Depends on child and wedding tbh. We married young so the only children we even knew were my cousins (youngest aged 6) and and DH's niece (2yo), I would have allowed any other children if we'd known any and but was fine, I barely noticed them there! But it was a french outdoor country and informal wedding.
DS came to 2 weddings with us when he was 11months and 18months. He was an easy sleeper so although we had to run after him during the day (although he wasn't invited to ceremony of the first one so he had a day being spoilt rotten by his godfather), he happily went to sleep in his buggy/in a teepee and we checked on him periodically and went on to be able to have a good time ourselves.
This summer we went to two weddings and the the DC were invited but we chose to not take them - one was a family garden informal wedding (a lot like ours) which actually would have been lovely to have them at, the other was a grand country house hotel and they would have been the only children there - everybody else decided to keep their kids away too!
We live on the other side of the world so not having the DC meant we really got to catch up with friends and have a great time focussing on that, and went to bed when we wanted, danced when we wanted - much better than missing out because of running around after our (admittedly generally very sweet and well behaved) DC.
We have two weddings, in the country where we live (so nobody to look after the children unlike weddings in the UK), one they are not invited but it's not til January so we've put aside the babysitting money already, the other they are invited but it's a big beach wedding round the corner from home with lots of children.
I'm not at all offended if my DC aren't invited to a wedding (they're now 2&5), and happily choose not to take them if I have willing and free childcare!