I've had similar comments and it's usually been helpful as the children aren't listening to you, but someone else saying something often brought them up short.
Responding with "yes, bad day" or "I'm offering them up for sale by this stage in the holidays" gets a smile response and relieves your stress and they know you're doing your best.
And don't do more than one explanation. They knew perfectly well they were unreasonable, they just didn't want to be reasonable. Just like adults too. I know perfectly well Crisps and Cola is a poor breakfast, guess what I'm currently eating
One quick explanation, then if they continue, don't discuss more. The more you discuss the more they think they can persuade you round.
I would have said "fine, it's your scooter, no one else takes it, if you don't then you've lost your scooter and won't get another one". And started walking. They very quickly decide that they'll bring it then.
7yo is old to have a strop about leaving the park. I'd suggest that maybe 2 hours is too long if they behave like that. One of mine could only cope with park/soft play for about 40 minutes at that time before he became over stimulated and got stroppy.
Another suggesting is have something for when they leave the park. So take water for the park, and have the juice in the car. Or an ice cream when they get home, if they leave when you say, or perhaps TV time or something.