DR TCB - As for the Mary Poppins' kids... Well, I think it's more a matter of the kids going through Mary Poppins and not the other way around. It's just the nature of the show. Jane and Michael Banks are essentially pre-pubescent children, and they need to cast pre-pubescent actors. So, when those pre-pubescent voices and bodies start to change, well, as they sang in "The Brady Bunch", it's time to rearrange. They also have three sets of Banks' children, so that's six kids right there.
Disney has also started having a general open call for all of the "young people's roles" in The Lion King (Young Simba and Young Nala), Mary Poppins (Jane Banks, Michael Banks) and The Little Mermaid (Flounder).