*Chessington, Legoland, London Zoo are huge treats for birthdays and as rewards for end of term, being good, etc.&
That might be the case for your children, but it wasn't for ours! We took our three out to every theme park, zoo, museum, nature reserve, National Trust property (preferably with an adventure playground) within 100 miles of our house, at weekends. In the summer holidays, if I felt like taking them to Chessington, Legoland or Thorpe Park one day, I did it.
I have done regular childcare of DGD from age 9 months, and I found it was a doddle, after three DC, including twins. We looked after both DGC last year for a weekend, aged 2 and 4, while DS went away with DDIL. DS has asked us this year to look after them, aged 3 and 5 for a week, while he and DDIL use some points for a long haul holiday. We have regularly had DGD for sleepovers and will have them both sometime in these summer holidays. I can honestly say that looking after two young children (and we all think DGD has ADHD), is a rest cure, compared to looking after DD1, with learning disabilities and challenging behaviour for the same length of time.
As for the comment about having to plan what to do, if looking after both while out somewhere and one needs to go to the toilet - how patronising? I find as a grandparent, looking after DGC, it seems like only yesterday, I was looking after three DC - things like taking one child out of several to the toilet hasn't changed!
TBH, if a parent asks a grandparent to provide childcare for two weeks in the summer holidays, involving travelling 6 hours to get there, I wouldn't have dreamt of telling grandparents what to do - I'd have left them to get on with it, as they saw fit, seeing as they managed to raise OP to adulthood safely?
I do see OP's DD as controlling, by organising a tennis club every morning for DGC. DH and I both wfh. If I were looking after our DGC for 2 weeks, I'd just take them to the park, or to town for a cake and drink or get a paddling pool out in the garden, whatever to get them out of the house, while he was on Teams meetings. I am able to use my own commonsense!