I suffer from this problem as well. Most importantly, when there is an open crack on her foot the problem is no longer dry skin, it's an open wound. When my heels and feet crack I put lots of Polysporin ointment on the area and put on bandaids, and leave them there (and fresh ones as needed) until it's healed.

Catty's suggestion of water would likely help as well, and I also take flax oil, vitamin E, A, and D to help with dry skin. I usually forget to take them but when I take them consistently for even 3 days I notice a marked difference.

The last thing I do is get regular pedicures, or do them at home. If there is no callous, there is nothing to crack. I have one of those pedi-eggs thing and I tend to my feet every night.

I hope this helps!