Well I can't help admiring you for thinking so far in advance, but I also hope it doesn't mean that the worry gets worse the closer Christmas comes.
I'm afraid I have no idea where you could get a cheap karaoke machine, but is that really necessary? How many children are we talking about?
Could you ask some of them to help with preparing the food you are going to serve?
If you have the time could you do a hide and seek game, hiding coloured sticks (or something else appropriate) around the house - the younger children get to find one colour in the easier places, and the older children get to find other colours in harder places.
Is there somewhere you could all go for a walk? If you have woods nearby give everyone a torch and have a spooky walk through the trees before bedtime, or if you live in a built up area couldn't you just go for a walk and admire all the Christmas lights?
I hope you get lots of ideas!