All this wrangling aside, some practical cheap fun suggestions for days in the holiday.
Park with packed picnic lunch and ice creams. Do you have an Iceland near your park? We do. I take a cool box and ice blocks and they choose their ice creams in Iceland. If not, set a budget for lollies out.
Get them to help make chocolate treats for the picnic. Rocky road or chocolate crispy cakes. Good bonding activity.
Making iced shortbread. Get a set of pipe icing colours from pound shop. If you can get cutters it's even better.
They can mix, cut out and decorate once you have baked them.
It's a nice activity to do at the end of the week as the children can make them for their Mum.
Make giant bubble mix. One part cornflour, one part fairy liquid ( must be fairy), 12 parts water, stir well. This make bubbles up to 7!!feet with the right blower but the blowers for the pound shop work too. Just ditch the bubbles in the pound shop wands.
Can you get a cheap 2 man pop up tent from Argos or equivalent? We did this a few years ago. We have them blankets and a picnic lunch. It was the activity for the week. It cost me £20. They loved pretending they were camping.
Roll of heavy duty polythene and a hose pipe on a garden fork. Splash a bit of bubble bath onto it and the can use it as a really fun run and slide activity.
If you tie dye, do it outdoors with children in bathing costumes or very old clothes. I know this from experience! The dye takes to the first T shirt quickly and if the are not all dipped together the rest are very light. Try putting all the T shirts in at the same time. The results are worth it. It's a fab activity.
I know the above relies on your mums garden, but may give you lots of peace whilst they have fun. You sound like you'll need your days to recharge if you are working evenings.
Best wishes, you'll have your work cut out, but it's a super opportunity to make close bonds and special memories. Take lots of pictures!