"..just got someone to look after us" is not always easy for a lot of families.
I do think that it needs to be made clear. Many people assume that if parents are invited, so are children. Certainly the first time I was invited to a child-free wedding, I needed it spelled out, as I'd never heard of such a thing. I didn't have family nearby, and although I had good friends and neighbours, asking them to care for young children for a whole day is a big favour.
I think it's completely up to the couple and if children aren't invited, then parents of young children may not be able to attend. No need to take offense either way, but some understanding is needed. It can be expensive and awkward to cater for young children at a wedding, and also expensive and awkward to arrange childcare for some.