Hello,
Working with roses would be a multi-year project- probably not well
suited to hold the interest of a 7 year old. If you really want to work
with roses, I would suggest crossing different species of roses (not
named horticultural varieties), since they would be hardier, would grow
on their own rootstocks, would display many different morphologies...
Another suggestion- get a hold of the book "Breeding Your Own Vegetable
Varieties" by Carol Deppe. There is lots of good info there.
Ron