It’s quite popular in my area to have two children’s bedrooms sharing a bathroom. Here are a couple ways that you can achieve this:
One way is for each child to have their own vanity and then through a door is the common toilet and bathtub/shower.
Another way is for the kids to share a common vanity area with the toilet and tub/shower in a separate area, so one can be having a shower while the other brushes their teeth.
For your own designs, account for about 7’x12′ for the top example and 6’x12′ for the bottom (although this one could be squeezed down to 5’x11′).