No don't so that - they would be poisoned very quickly as they are even more sensitive to ammonia than the adults. If you haven't got a cycled tank to transfer them to you are going to have to leave them in the main tank and the breeder box will get crowded pretty quickly. Get some plants in your tank if you haven't already. Live or not, they will give the fry places to hide altho live is better as they have micro-organisms on them for the fry to eat when they are hiding. Sure, some of them will get eaten, but the fastest and fittest will survive.
And remember, your platy mum will be giving birth again in 4-5 weeks time (and so on ad infinitum) so what are you going to do with 30+ fry from every batch? Unless you are planning to buy and cycle a new tank every time where are they going to go? I allow nature to take its course in my tank - the fry take their chances with the adults and soon learn how to hide. I get between 3 and 6 survivors from each batch and believe me unless you've got space and money to house them all that's more than enough.
Good luck!
