My two cents...
The difference will be marginal. An hour of extra battery life, maybe 10% faster CPU (and only in optimal circumstances), et cetera. So you'll have to weigh that against the price. If you can get a really, really good deal (meaning not just $150 off, but maybe $250 or more) then sure, go for the 2015 model.
However assuming your kids are in school, you may be able to get educational discount. That, plus the fact that new models will have a higher resale value, will probably make it more interesting to get the new ones. And the new models may come in colors...
As an aside, with kids you may also want to give them a school bag with a specially padded laptop compartment. Speck also sells plastic cases/covers and pretty quickly follows up on the release of new models. A minor bump will then not dent the MacBook but just scratch the case. And with a case, they're much more recognizable.
Check if your home insurance covers laptops outside of the house. Might be worth it to extend that coverage. Much cheaper than the in-store insurance offers.
And put a different sticker on their chargers.