On point 1, you were just as guilty in giving your dc a say though. She suggested something that the 8yos would enjoy, you said no because your older ones wouldn't enjoy it.
It can be hard to find things that that range of ages all enjoy -I have 13 through to 6yo, and there are some things they all enjoy. But it tends to mean imput, and I'd guess she was thinking that way you could sit down as friends and chat rather than chase after the children. I tend to find that if it's something suitable for all, then either the younger needs a certain amount of help, or the older finds there isn't enough.
If I'd been you, I would have probably left the older two at home. Or if they'd wanted to come, happily given them the entrance money (because they'd either be too old to go on so you wouldn't pay entrance fees or if they were allowed on, they'd be just as happy doing that as my 6yo) to spend in the cafe and let them feel grown up having a drink and a cake, and they'd quite enjoy reading/playing on electronic devices etc. in the corner. I have done that at times, when friends with ones ds' age suggest meeting at soft play.
Point 2: I agree with your friend. I wouldn't tell my dc if it was a "maybe/perhaps" situation. I wouldn't be cross with you for telling yours, but it's much nicer for them to be excited because they have a nice surprise, than disappointed because the person can't come.
Point 3: Does depend on how it came up. Your dc say "you've got a great iphone, I wish I had one" (or whatever) she says "you'll have to ask your mum". Would be a perfectly reasonable discussion. Is your dc are asking "How come she's got a better phone?" and she says "pester your mum rather than me..."
I used to get a lot of comments until I upgraded my phone last year. I said it wasn't a smart phone, it was a "stupid-old-phone"-but it was perfectly modern when I got it... second hand in 2000, didn't even do predictive text. Often people asked when I would upgrade and things like that. Didn't worry me. When I got my new phone (still not expensive) I've had lots of comments about "joining everyone else in the 21st Century". 
As a child I didn't like chips, still not bothered about them. I wouldn't have asked for anything else, nor would I have moaned, but I wouldn't have eaten them, particularly without a drink I would find them uneatable.