from what I've seen raising three batches now, cornish cross begin a pretty steep downward health slope after about 6-7 weeks. when I harvest them it's kind of putting them out of their misery, and that is with restraining their food and keeping them moving. I know there are folks who have gotten them to grow out more but I'd kind of have to agree that you are working against a strong genetic predisposition to keep them as "forever birds". you might be setting your daughter up for a sad road. I have put mine on the typical 12hr on 12hr off regime and it's hard to imagine restraining their food even more, they basically sit there and beg and then climb all over each other once I put the food back in. if left unfed long enough they start eating dirt, anything to fill the void.
as for sexing them, it's possible in the first couple of days based on their wing feathers but after that it's a hard call till they are older, but only by a few weeks. by week 5 or 6 or so it gets pretty obvious. If I had to throw out a guess, I'd say the one in the foreground is a rooster and the one in the background is a hen, based on subtle differences in their combs. the hens combs are slighter pretty early on. expect them to have something like 10X the normal growth rate of other breeds, it's kind of mind boggling... they live fast and die young. they are essentially made for one purpose.