That is rough.
I know how hard it is to have to break bad news like this to the little ones.
This late in the year chicks, even from the hatcheries, can be hard to get. So Spring might be your best bet.
Might I suggest finding some local breeders. It might be more fun for the kids, and certainly easier on you to get them right from the source.
In my experence breeder quaility birds are well worth a drive (I've traveled over 6 hours one way LOL)!
Unfortunately if you're looking for an assortment of breeds and wish to get them at the same time you often cannot do that with a breeder Breeders seem to at most have only a few breeds (and yes, I've gotten birds from serious breeders, usually one breed at a time.) However often feed stores or farm bureau type stores will get an assortment of breeds in. At least in this case, although they have gotten them from hatcheries you get to pick from among those that survived the trip and are less likely to die.