Much as we would like to think otherwise, children can be hard to entertain, especially if you are under a tight budget and the weather is wet and cold!
Since you are crafters though, you have an advantage. There are few children who don't enjoy making things especially if an electronic tool is used..
I remember playing with cut out shapes as a kid, painstakingly cut out by hand from cereal boxes. This is a more modern version, made easier by a cutting machine, although if you have to, you could do it the old way, it gets tedious rather fast. See the Tutorial here to see what I mean.
With a little imagination you could incorporate not only different geometric shapes but things like animals too. You do need to cut quite a few, so I would recommend either coloured card or let kids loose with markers later. You can vary the sizes and vary the angles of the slots too. If you are very particular you can make these very precisely - I just eyeballed the demo one.
Make some simple stencils for the children to use - provide cheap paints and sponges to apply them. Don't forget to spread lots of protective paper around too, and use old clothes as coverups! If it is age appropriate, they love to sprinkle glitter on it all too. Expect to find glitter everywhere though for at least 6 months.
Make your own version of photo cubes out of card. Make a cube box from your favourite design. Add squares of acetate to the sides, glueing only 3 edges on each. Trim your photos/drawings to fit and slide in.
For children learning to tell the time, make wristwatches out of paper or a clock out of card. The clock can have moving hands if you use a brad to hold them in place.
Remember those sewing kits which were just card shapes with holes to thread? Why not make your own? You can use pictures from magazines or comics, stuck on card and use the Rhinestone feature if you have it to cut the holes out.
Paper beads are easy with a cutter, accurately cut to the same size and so fast! Use old papers or scraps that are hanging around in your stash.
Encourage your children and grandchildren to craft, and you will not have dull days! I have yet to meet a child that did not originally like paint and glue, paper and card, scraps of fabric, old buttons.
Older children can, by the way, make buttons very easily from polymer clay. I made some ages ago and put one on a homemade case for my iPad, it is still just fine!
Older children may like to try salt dough modelling. It is very cheap to make the dough but requires long hours of slow baking or some very dry, hot weather to bake it solid. Once finished it can be painted and sealed with varnish.
So have a go at removing the youngsters from their gadgets and challenge them to see what they can do, it may surprise them more than it does you!