I'm just wondering how much walking she's really going to be doing? With both her feet in casts, i don't see it being much. I'd probably just plan on getting an umbrella stroller and have her in that for most of your outside trips.. then let her walk inside the house so that her muscles don't atrophy too bad.
When younger dd broke her leg (at 4 yrs old), for the last couple weeks of being in the cast she was allowed to walk on it, but it was so hard for her to be able to do that, I just kept her in the stroller.
Not sure why she's in casts, but are they recommending she stay off of them as much as possible? or walk on them as much as possible?
A cheap alternative to a walking boot would be to velcro a pair of flip-flops to the cast. Just cut the straps off, and use the sole. Can cut slits through the sole to wrap the velcro around.