When I worked in the nursery, we had a rule for parents: Don't leave until the kids are happy. We took crying kids to their parents, plain and simple!

Nursery workers can't be expected to know what the kids want, only parents know how to calm their child the best way. Your sanity is more important.

Also, we've found that distractions are a great way to get the kids to smile. We got out the bubbles and it seemed to work every time. Music and snacks are also a thing kids looked forward to.