So your friend had a nice day out and documented it on social media, how is that unreasonable? You just sound jealous tbh! She didn't mention prices and costs until someone else mentioned in the comments and she responded with an actual figure, which might be high but you have no idea of it was within her budget or not!
Taking six kids anywhere is never going to be cheap, but why should it only be allowed for "special occasions"? I am by no means well off, but last weekend I took my DD, my two nieces and nephew out for the day, just because they were all at my house and it was a nice day so we decided to go an do something fun.
We walked into town, stopping en-route for an ice-cream and a round of mini golf on the little course in the park (I had a voucher for a 1/2 price family game) ; we then went to a nice locally owned Italian restaurant for lunch (where they have had the same daily lunch deal for at least the last 15 years of 2 for 1 on any pasta/pizza); then we stopped off at the toy shop and I let the kids choose a toy each (within a £ limit and there was a sale going on); on to soft play for an hour then back to my house where they built a pillow fort, got into their PJs and watched a movie until my siblings came to collect their children.
It cost me maybe £150 but the money was worth the fun memories we all made - just because. We didn't have to just wait till a birthday or Easter or whatever to have a nice day out ... it was nothing special, just a couple of activities, lunch and a movie and I could have done free activities only or cheaper activities, but equally I could have done much more expensive activities too. The difference being that I wasn't going through the day with a mental cost tally, we just did things I knew were within my budget and I knew either had good deals and or I happened to have vouchers for.
Yes I posted about the day to FB because I wanted to share the pictures and memories with our wider family/friends (some of whom are abroad and so don't see the kids much) and also to document the day for future reference. If I thought another friend was judging me and gossiping about the fact I was daring to have a fun day out with my family that shock horror! cost money and post about it on my own social media, they'd be getting dropped straight away.