Oh well.

I had this idea about creating a child character. She will be about the age of fourteen - fifteen, so. I was just wondering what a child can't or can, and an advlt can or can't. I want to make this roleplay more original. I will be writing a fanfic about her.

I have doubts: I'll probably make a Bosmer or Breton. As a Breton she will be a full mage, and as a Bosmer she will be a full mage but I will be replacing destruction with marksman. So I don't know yet, just try to come up with some roleplay ideas. I'll appreciate.

Oh, and another side question: how can I make sure that my character will be level 20 as maximum?

Well if she's going to be 14-15 there wouldn't be that much of a difference physically, just lower endurance/strength, perhaps intelligence. :mellow:
She should only go to the higher class inns, you don't want her to get mixed up with thugs and drunk people (unless this is a moreover evil character).

As for level 20 being the maximum, just take 3-4 majors that you never touch.
Each race/class is different.
To calculate it just take the majors you're using and their level at lv1. Each 10 skill+'s thats 1 lvl.
Make sure that once the majors you use all reach 100 you will be lv20.

You have doubts about what?
I feel with either a Bosmer OR a Breton, you'd be able to bring across the youthful innocence quite easily. Why, my own character is only 18 at the start of Oblivion, and when I started writing her, she was 13.

Of course, the final decision is up to you, but here's my 2 septims. I'd make the Breton not use Destruction either, but that's just me. The Bosmer would be incredibly fun to play. Ask Acadian, magically enhanced archers are absolute beasts (when you get to dictate the fight, when you don't, it gets hard).
However, I prefer Mages at the moment. So for that reason, I'll suggest you make a Breton. Plus, that way Helena will have someone to talk to that's near enough her age! I'm sure that'll make her very happy.

Andyalam, I believe the opposite. From an early age, Helena Aurelie was forced to beg and steal to survive. Through this, she befriended a Khajiit, an Argonian and a rather quiet Dunmer boy, all in the same situation. Together they forged a very close friendship and shared the troubles of life on the streets.
I feel this added masses to her character, and I could see it doing the same with Esmée's character. However, it depends on her financial situation I guess. Helena didn't have any choice.

I cap my character at 20 and like that very much. There are several approaches to the mechanics:

1. A normal character who never sleeeps after level 20. Ok I suppose, but I couldn't deal with the RP of no sleep.

2. A normal character who levels normally to 20. For subsequent level ups, use the command console to bump her to 19 right before level up. Requires PC.

3. Control the max potential level by selecting a number of majors to not use as andyalam said. If you do the math, you will see that 20 is awkward. Maxing only 2 majors while ignoring the rest gets you to around 14, then you can read skill books and practice some of your 5 'never use' skills to get up to 20. This is good in that you don't have to sweat inadvertant level ups in your other major skills. Maxing 3 majors while ignoring the rest is tougher because you have to be very careful not to end up at 21 or 22 and really have to watch out for those skill books and other things that can give you an unexpected level to a major skill that you don't want.

In selecting major skills to never use, don't overlook the impact on attribute gains. Since my character ignores strength and endurance totally, 'never use' skills governed by those attributes make sense for her - skills like blunt for example. Also, attribute gains become somewhat more important when you have less levels to accomplish them. Take care to max any attributes you feel you must by level 20. Ignoring several attributes not only helps focus and define a character, but it also makes building key attributes easier.

4. Rethink level 20. Level 23 is easy to nail by maxing 3 major skills, and gives you access to perfect amber/madness stuff from the SI if you like that (I don't, so I stop at 20).

If you want a mystic archer for example, you might consider maxing marksman and restoration as majors. As a young character, you might consider using security as a major freely while you build up your magical abiity to open locks. Once you have your level sorted, switch from secrutity (major) to alteration (minor) to open locks. Here you would be maxing 2 and almost maxing a third major skill. Just a thought.

As far as a Bosmeri mystic archer or a Bretonic pure mage, either is great fun and can be very deadly. So much so, I recommend employing some weaknesses to tone them down into glass cannons, such as low strength and endurance.

Yeah, I can see how that can add alot of depth.
I should've included more than just "unless she's an evil character." I was assuming Esmée's character would be one of the stereotypical scholar mages though, my bad.

I see where you're coming from. It's hard to imagine a Mage-type (as Helena is) stealing and living on the streets. But when you think about it, it's perfectly possible that they had a difficult childhood. I'm rather fond of characters that are thieves of necessity. I feel it adds an element of vulnerability as they're being forced to do something they wouldn't normally just to survive.

I just came across http://www.tesnexus.com/downloads/file.php?id=20271 earlier today that lets you play as a teen age Breton. It might be exactly what you need.

Curse my lack of memory. I know there was a forum user here who had a character called Angel. She was a child, and quite a successful character. However, I can't for the life of me remember whose character it was >.< (sorry if she's yours and you're reading this).

That was bobg. As I recall he used the console to change her height in order to make her appear as a child.