what's the easiest way to kill the child of atom people ?

...shoot them?

I am not an expert on them yet but my encounters went bad first time due to high radition dammage but small phisical damage so stimpacks and radaway went down fast.

Alternatvely takeing them on with no armor but wearing a radiation suit and taking say some rad x well... Give that a shot. You can get a radiation suit in general suplies store in Diamond city.*

* depending on level / supply spawns of course

Haven't tried, but i'd imagine Hazmat Suit would make you pretty much immune to their gamma weapons.

Edit: Well, now i tried in Crater House, and their weapons indeed were useless against me.

i first came across them at the museum of witchcraft, they're armed with gamma guns & plasma grenades / mines which send radiation damage high

Yeah mate get the Hazmat suit as they actually call it.. You get the idea. You will take some damage but a few stim packs rather then having to dose yourself with radaway everytime you take a stimpack saves lots of caps.. The suit is not very expensive and very useful

Some of them on higher levels are absolutely insane. Forget raiders, gunners or super mutants, these guys hurt a lot. Not only do you have to avoid the radiation damage, they also pack nuke grenades!

I haven't seen a single one of them so far, only their dead bodies when I went to visit a certain junkyard. Is that unusual? Also, why are children of atom attacking people? Weren't they a peaceful group in FO3? I suppose they could have extremists like every religion does, but there's no context for the change in behavior.