When the Grandkids go to school

If they are local, as you say, then no need for an overnight stay until they are a bit older?
We see our DGC either once a week or certainly once a fortnight ( by their invitation) our DC asks if we would like that and if we are free.They pop over for a few hours, or less and we do an odd pick up or take to school now and then.
Six weeks is a long time!
Does it need to be formal? Would you be happy if they dropped in for a cup of tea and a chat?

The thing is, by making it just the school holidays, it appears to be just wanting their child care.

I'm sure they have enjoyed looking after the GC very much but they may now be looking forward to doing other things now.

You sound absolutely lovely,It is so thoughtful of you to think ahead like this. I agree with other posts where they suggest talking to the GPS,let them know how much you have appreciated them and ask how they would like to go forward,making sure they understand you needing time with your children. It will work out fine and I am sure your thoughtfulness will make them feel very valued.

Thanks all - it's definitely not just about childcare - I can work from home so that's not an issue at all - I'm literally trying to find a way to make sure every one still gets time with the children (and the children still get to see their gp)

They don't stay out very often as we like them at home, probably once a month on average they stay out - so one month they'll stay at my Mums, then the next at the In-laws (me and the husband can go out then which is nice as we haven't done much until recently)

It kind of does need to be that formal, not so much for us as for the gp - they have quite a lot on so it means they can plan around it if that makes sense? Also, I have no problem with people dropping in for a cuppa, it's just not the same for them is it? X

Why don't you ask the GPs? That way, they will still feel involved.