I wouldn't take a deposit at first meeting. I always insist on at least two visits...the second can be a contract signing and deposit giving meeting. But I feel families need to go home and thinking and come back before making such a big decision. If they are just feeling stressed and grabbing what they can find then they are more likely to over look things and make a poor choice.
I would let them know you were not happy with the second family to come through and let them know if they are still interested they are welcome to a second visit. In the least if they find they are unhappy with the daycare they secured they know they can potentially come back to you.