When the Grandkids go to school

I don't see the need to make a formal arrangement. Just see them when you can, include them in the occasional outing etc.

Some of my GC are at school now and I pick them up after school and look after them till Mum finishes work.

Oh yes, we changed completely when the GCs went to school. From my pov: we expected it to change, and we all talked about how best to manage it.
So rather than 'offer' something you've decided on, talk about it, ask ideas from both sets, and arrive at a family solution.
We do fewer hours in term-time, and whole days + overnight stays in the holidays (and take our own holidays during school term time)

Sounds like you've had a good relationship so why not ask them what they feel would work for everyone?

As Tanith says - they might surprise you and say they want to travel , or pursue other things so a scaling back on childcare duties and reverting to being grandparents might be just the ticket.

I'm sure you'll find a happy medium .

It’s surprising hard on grandparents when their GC start school, especially when we’ve looked after them since tiny babies.

I’d suggest that you recognising this is a good start. Why not have a little family get together, acknowledge their help and how this will affect them and discuss what continued contact there could be. It’s as much about maintaining the close relationship we have built up with these previous children as anything else.

So talk to grandparents and see what can be arranged. I still pick mine up from school on certain days and hold onto them until 5.00ish. Suits everyone.

I think on the whole GP realise their time with grandchildren will be severely curtailed once school starts and how lovely that you realise they will miss that time. Unless they live close enough to maybe collect from school for tea now and then I think what you suggest is a good plan. You might be surprised to hear that a lot of GP actually breathe a sigh of relief to get more time child free ?