I know how you feel.... When first opened I took on any age and stage. So I ended up with kids from 11 mos to almost 10 years old.
What I learned (and this is just me) is that I can do the little fellas.... and I can handle the big kids.... but not at the same time! I just found it exhausting.
So to answer your question; I would also see if I could "stick it out" until the end of the school year, and consider a lesson learned.
If you just cannot, then you may as well tell the parents sooner rather than later.