It does feel daunting minimo, so don't feel bad about that. I agree, don't expect a tidy home and also don't pack the days too much. Children, even boys, like some down time.
If you don't have toys ask their parents to bring some favourite ones with them. Ask them to remember to pack anything the children take to bed. If you have a suitable garden some outside toys are good, or consider buying something like garden skittles, good for all ages and less damage than a football. Otherwise think simple - helping with cooking, making a den in the garden with a blanket or three, go to story time at your library, see if there are council or Church run activities they can join, a local museum you would all enjoy. Let them help with dusting and hoovering, mine loved to help with the garden, little ones are closer to the ground and are useful for closely supervised weeding.
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Ours liked to see the animals at the local garden centre, I liked the sit down in the cafe afterwards
. If your OH has health restrictions get him to be in charge of all things that involve sitting down, so books, jigsaws, colouring, simple construction, supervising tv watching and any technical stuff they are allowed. Keep food simple, prepare as much as you can in advance.
You can do it! The children, even at three, can understand their grandparents have physical limitations and will adapt if explanations are given. If the three year old uses a pushchair at times make sure their parents being one. Even if he walks the ousxhair can carry coats and 'stuff'. Yes, you will be tired but it will be so much fun, there will be cuddles and laughter and the boys will remember their week with love.