This is a difficult situation - I can understand why you are upset :/
But in saying that, I agree with some other posters that at the end of the day it is their wedding. Perhaps they felt they couldn't say yes to some children and not others? I only suggest this as I am getting married in 8 weeks and my fiance and I felt this way. We have toddlers in our ceremony but have had to say no to them coming to the reception which we felt really bad about - BUT if we said yes to them, we would have to say yes to everyone - and everyone would have been 56 kids under 12! Our nephews and nieces who are in our bridal party - have not been invited to the reception. We do have 4 newborns coming..which we are totally ok with....but it was not a easy decision to not let other kids come. In some, ways I can understand from their point of view of saying no to every child would be easier than saying yes to some and not others. Not saying I agree, but as I have been through a similar experience recently - it is not a easy decision.
I hope the situation works out and you get to go to the wedding