CharlesV

The easiest way should be Consular Processing. They shouldn't need to be outside the US for very long -- just go interview, get the immigrant visa, and come back. Just make sure they have no potential bans first.

I don't know what the risk of leaving would be. They are underage so they will not have a ban for overstaying. Them remaining here illegally is more of a risk although I think deportation of children is rare.