It's a strongly held belief of the JWs that celebrating the festivals of the church is wrong. IMHO the owner has the right to impose that on his daycare centre because it's his belief, to the same extent that you have the right to go to church - one of the elements in his promise to parents may be that the kids aren't exposed to these 'heathen' practices. The justification seems to lie in a strict application of the 'sola scriptura' principle: that we should only do what the early church is recorded as doing. They didn't celebrate Christmas or Easter, so we shouldn't. (They probably celebrated Passover and Succot, but that's another debate - Luke in Acts occasionally refers to some of the feasts).

Another basis for this is to argue that Easter, for example, is a lightly Christianised version of the pagan spring festival, marking the return of the good weather and the start of the fecundity of the new growing season; this is enhanced by the argument that the name 'Easter' is that of a pagan deity - at least Christmas has a Christian name, although corresponding in time with the pagan celebration of the winter solstice.

Given these arguments I have no hesitation is saying that you need to let him have his way in his daycare centre; in the same way as you would object to someone celebrating the rebellion of Satan if you were running a Christian centre!