Try to talk to the mom about the schedule she is using. There is a modified delayed schedule that some parents use based on the idea that the more mature the child's system is the more effective it is. A good example is the MMR. My kids all got it at 15 months rather than 12 months based on my oldest three getting it at 15 months in other provinces. When my fourth came along here in Ontario they wanted to do it at 12 months because they said even though it is more effective if given at 15 months because of kids in daycare they do it at 12 months figuring the lower effectiveness offsets the delay. Since my child wasn't in a centre and everyone else in my daycare was getting it at 12 months I went ahead and waited till 15 months for her.

I like the idea of telling the mom in writing that the immunizations need to be up to date by said date or the contract will be considered terminated by the parent and deposit forfeited. Make sure to cash deposit now so it can't be cancelled by parent.