Try to understand Hierarchical Partial Reconfiguration(HPR)

I'm not entirely getting what you are suggesting here, but once you create child regions/partitions, they are by their nature isolated from the parent, so you can either reconfigure the children themselves or reconfigure the parent, which in turn affects the children.

I think what you could do is have the default personas for parent1, child1_1, and child1_2 be your larger "bit1" design, distributed between the parent and the two children instead of just one larger partition. Then you could PR the children to alternate personas bit2_1 and bit2_2. You will lose some resources due to the fencing of LL regions (which can't be avoided), but you are still essentially splitting the larger bit1 design into smaller child "chunks".

Maybe a better explanation of your idea here would help.