Dear Union, New Jersey:
I am sure that you are not aware of it, but in order to get governmental funding (federal, state or local) most non-profit daycare operations have a requirement that the daycare provide care for a certain number of children for a certain number of hours a day. If the daycare does not do that, their funding is cut. If the funding is cut, then they cannot pay teachers, rent, etc. and therefore, they will go out of business.
That means that if a daycare has a child who consistently cannot stay the minimum number of hours, they daycare center will maintain waiting lists of children who can attend so they do not lose funding.
That is almost certainly why the daycare center wants to expel your child. The agreement that you signed with them probably required you to keep your child there a certain amount of time. You would have problems bringing a legal action if you weren't in compliance with the agreement.
The result can seem harsh, especially if you want to spend a lot of time with your child. One thing you might consider doing, if the daycare center will allow it, is volunteering for your child's class so you can spend more time with him or her, but do not have to take him or her out early.