I wouldn't make fudge with children that young, but peppermint creams are something they can really get involved with themselves (or you can flavour the mixture with anything else - we used to make lemon creams)
You can also use silicon moulds to make chocolate shapes and melt the choc in the microwave. We used a mould that made mini chocolate spoons - if you do some in milk, some in white and some in dark choc they look good and are lovely with a cup of coffee or hot choc
You can also drop big buttons of melted choc on baking parchment and then sprinkle dried fruit, nuts or anyhthing else on top. You can do this in huge bars, mixing the chocolate like slabs you seen in hotel chocolat but the big buttons are easiest when the kids are small and the inevitable irregularity looks good.
Finally, pickled red cabbage is ridiculously easy - you could shred the cabbage then let the kids just pack it into jars and pour spiced vinegar in. Get the kids to scribble on a few labels and stick these on the jars too.
I bought candle paint in flying tiger and pillar candles in ikea. I haven't tried it out yet but the plan is to let the dc make decorated Christmas candles
Hyacinths and other bulbs can be grown fir Christmas, starting now. I think lidl or aldi sell kits.
Cut out thin circles of fondant and decorate with coloured icing (dr oetker do a kit with little tubes), Christmas sprinkles and silver balls. Let them dry and store in airtight box and add as toppers to fairy cakes (buy if you run short on time to bake, or bake now and freeze) at the last minute. Don't freeze or refrigerate fondant as it sweats. Biscuits and cake freeze fine but you might need to let people know so they don't freeze them again
Orange and clove pomanders. I recommend poking holes in the oranges first as cloves are hard on fingers
Half melt choc in ziplock bag in microwave, and leave a few mins to let rest of choc melt. Seal bag and snip corner. Pipe onto parchment or baking paper, or use a silicone mould and cover the results liberally in sprinkles. Cellophane bags and ribbons.
I don't know what to call the cube block of paper you can get for phone notes? Let DC decorate the sides of the block with sharpies.
We had a kit of beeswax sheets that you soften with the hairdryer and roll into candles. Smells lovely and very tactile craft for little hands.