I would flat out call and ask either the mom or the bride about it, I wouldn't make an issue of it, just matter of factly ask if the children were included, by the way age 18 is not a child to me and by the time a child is 14 they should know how to act at a wedding so what could the problem be???? Maybe the food for the reception, if that's the case tell the bride that you all will attend the wedding but then your children could either leave or all of you leave at that point. Sounds like a food budgeting problem to me?????
Doing that would be making an issue out of it, making the OP look like one of those guests. She already knows her children weren't invited and calling the bride or mom to ask about it will put them, and the OP in a very awkward position.
OP, I agree with the other posters who said that they probably didn't feel it was fair to invite your 18 year old and not your 15 year old. It so hard when you have a limited budget, or space to figure out who can come and who can't. Maybe they just thought not inviting either of your kids would be more fair to them than only inviting one.