It's a hot topic the kids/ no kids wedding thing!
We had no kids apart from our nieces. We didn't have kids of our own at the time and wanted our wedding to be a grown up party, with lots of drinking, dancing and late night. DH relatives all have huge families If we had invited all the kids as well it wouldn't have been the same kind of do. Half the guests would have had to have left at bedtime. We would only have been able to have invited half the number of friends we wanted as kids counted towards the venue capacity and not space for everyone!
We spoke personally to our closest friends who we really wanted to be there and thought might have childcare issues, to explain our decision. They were all very understanding and respected our decision and actually enjoyed the opportunity to have a child free evening. One couple hired a nanny for the weekend - a massive expense we never expected of them, but they decided to use our wedding as an excuse to treat themselves to a weekend away from the kids with good friends! Although some people (slightly more distant friends/relatives) declined the invite based on us not inviting their children, TBH they were people who it'd been nice to see but weren't top priority guests 
Although there are many reasons why guests feel 'disappointed' with their wedding invites - too far away, awkward location, expensive accommodation, a weekday, a bank holiday, already booked something that day..... or no kids.
So if you feel you can't make it without kids politely decline. If you feel you need to then say it's due to childcare, but DON'T ask the couple if you can bring the kids!!! That's basically asking for 3 more invites - and asking them to change the dynamics of the day, especially if no other kids. You might want your girls there and like a family do but they have decided not to invite them. Don't get worked up over it.