Let them age and become more viable members of the team with power sets of their own. Gives the book new perspectives and different avenues of drama and may make it more relatable to younger readers. I'd say somewhere in the range of 12 to 16.
Although I agree with Owl below (they are trapped forever as kids) I would like to see this. 16 year old Frank and 12 year old Val arguing in some universe Frank created for her with Moon Girl for eternity about who's smarter while they miss the big picture.