I would tell them that until the ft spot is filled, the extra days can be used. Once the ft spot is filled, it will go back to 2-3days a week. This allows you extra income for now but won't stop you from filling the ft spot

Also, if you wanted to be fair to both moms, I would offer them alternating days. They still share the spot but each one comes 5 out of 10 days instead of 6 and 4.