This is so tough because you need to have payment. You don't want to throw out threats or new policies until they are paid up.

The fact is... if they haven't been to you in 3 weeks then they probably have alternate care and won't worry about a threat of not giving care until they pay.

You almost need them to think you'd "like" payment and you're looking forward to seeing them the next time the child is booked in. Then when they don't show up with payment you can refuse care. No time to find an alternate in that case.

You still may not get paid... some people are just like this. but at least you've screwed up their day.

I would hit them with notice as soon as you receive payment and that way you're up to date and the deposit will pay for the last few days of care.