Family days out that won’t break the bank

The beach
The woods
The hills
Bike rides
Nature trails
Picnics outside
Tent in the garden

Just be inventive

In London the major museums are opening, I assume it's the same around the UK.

Usually during school holidays, half-term etc. the queues and crowds are horrendous but in these special times you now have to book a time slot, which suits me down to the ground.

Your GC are of such varying ages I doubt you'll find activities that suit them all, at all times! A family picnic would be do-able.

One half term grandkids spent several days planning a picnic, writing the menu, writing a shopping list for required supplies, doing that shopping, baking a cake (battenburg! their choice!), making sandwiches, packing everything up including tablecloth and napkins, and then we drove them to the spot they chose. All very memorable and honestly if we had eaten it in the back garden that would have been fine, it was the planning and preparation they still remember. Eldest was 13 at the time.

Making an obstacle course in the garden, then organising the Olympics is also fun.

Could you do an overnight camp trip ?
A boat trip at sea houses if operating?
Just a mad thought could they learn to paint a room ? Or the garden shed ?
Is beamish open ?

Honestly, anything that is outdoors right now. We love going for a walk and grabbing a coffee.

As this was in July, I imagine nanaelaineb managed to entertain her charges!