I think it is fair! I went to San Diego a few years back along with my bosses (husband & wife) and 2 others. They paid for the working days (2) as per normal, but the one weekend day we were there, we didn't get paid. They also didn't pay us for the time we were travelling. Just 2 days regular wage.
We did have free time after/between the lectures, although not a great deal, we could still do what we wanted (and I am talking just enough time for a meal or couple of shops).
All flights, airport taxes, hotel rooms, meals, drinks were on them. All we paid for was what we used in the room in terms of movies hired & telephone calls.
Again, they are paying you your working week & paying your colleagues their working week. It isn't their fault that you can only commit to part time & they do full time, nor will it be your fault when you get several months maternity leave & they don't!!
Yes, you have to find childcare to cover your time away but that is not their fault when you decide to attend a voluntary function. When my workplace arranged a similar trip & I couldn't afford/get childcare, I simply didn't go, but I certainly did not think the onus was on my boss to pay me more than I earnt to fund childcare costs when they would have been paying all the other related charges too.