I am a pro mechanic. Hate to say it but much harder to find a solid bike for under $500 these days. I consider Walmart bikes unserviceable and borderline unsafe.
If you could find a previous years' model on clearance for the 10 year old, I had one for my son and it was a very decent bike:
https://salsacycles.com/bikes/timber..._timberjack_24. But, beyond your price range.
For the 13yo, she could probably go with an adult size at this point so there may be a bit more longevity with her bike. At that budget, you're best off (IMO) looking for something from Trek / Kona / Giant / etc. on sale somewhere. With some digging you might be able to get under $500.