Aside from possible disruption during a service, I do think it may often come down to money. I'm sure many people would like to invite children, but if there are a lot, it is going to bump up the cost considerably, or else mean fewer of the adults the couple would like to invite. Heaven knows weddings don't come cheap.
There were lots of children of all ages at a dd's wedding last year, but we were extremely lucky in having a lovely big venue to use for free, since it belongs to a generous relative. There was loads of space for children to run around in, and games equipment laid on, plus babysitters and a sleeping area for later. It was a lovely, relaxed and informal occasion, and we were also blessed with fine weather, which made the running around outside possible.
However, I hate to think of what the same would have cost if we'd had to pay for a similar venue with as many guests. I'm sure the whole thing would have been scaled down considerably. Should add that a number of dd's friends with young children did not bring them, although they could have, since they welcomed a break with a lie-in in the morning! Though of course not everybody will have anyone willing or able to babysit - far from it.
Army horses loose on London streets
Last weekend, in Rutland, the first statue in Britain of the late Elizabeth II was unveiled.