If this is all a valid post, part of me is wondering whether these kids are particularly difficult or a handful.
I remember thinking I didn't want to go bloody anywhere because all I could envisage was a shit show with my 'spirited' child. And I decided that exact same thing - you don't want to wear the coat. Fine. You get wet, don't moan or playtime ends and I won't do that again. I understand that approach . And it's one you might see being applied to kids who are a handful. But I may be wrong.
If your garden is full of hazards and fall risks, that's just exhausting for a mum of 3. It will be a bin stop no don't, stop that, come here, especially if they are full on. I got a feeling they could be 😬😆.
My child would never do seed planting ever! But getting messy, playing with balloons, water balloons, sand, tubs full of sand in the garden, making mud pies, all that yes. I'd say most kids that age will have more fun doing this. Can you clear every single hazard in your garden, are you ok if flowers get trampled. Can you just cover that off so no one has to say no no constantly.
Remove all ornaments or things that will be knocked over in the house. This removes huge stress for mum, your son and you.
Get balloons, get music sorted, get something's that covers up a table so they can create mess. Get play doh. Get crayons and paper. They're hopefully going to love that - if mums ok with it.
If it's easy and stress free they will love this but only if you can be bothered. If you can't that's ok. But kids are not usually like Enid Blyton characters so much these days I feel.
I sense you're worried you won't do it right. Are you worried you will be punished by the son or wreck it? It's not your fault if the kids have a bad time but the above will help. You might want them gone quickly if they're exhausting. That's absolutely ok.
If you're happy to do the above then maybe ask son what he thinks about the suggestions. Ask about any allergies and food or drink requirements in case she's particular about sugary drinks etc. Then the rest you just leave them to tell you.