Year 7 Maths Attack!

The school I'm at has a primary school part, which, though attached, isn't very integrated with the larger school. I've been planning an event where a small group of my year 7 group will run workshops in a year 4 numeracy class. There will be four activities all going on at once, and the year 4s will rotate to each in small groups. I'm hoping that my year 7s will be up to this, so that I can spend some of my time taking photos and not spend all of it helping them out. Anyway, I thought it would be nice to share the activities I will be doing and the resources that go with them.

1. Numerical twister: Like normal twister, but each spot has a number on. The caller calls out a number and you have to make a sum that is equal to it. Can increase difficulty by saying that the sum must include 3 numbers or must include a multiplication.
2. Fractions and Decimals top trumps (google docs seems to mess it up a bit, but just click the link to download the original): Top trumps cards with a combination of decimals, fractions, large numbers and fractions of amounts. This is the one I see most potential problems with, and I will need to train my year 7s on what to do when the year 4s don't know how to work it out (hopefully they will be able to show them using a mini-whiteboard).
3. Estimation game: pupils have sock puppets on (making it a bit more appealing, but also to make it harder to feel out the answer). They each grab at some beads in the middle then try to estimate how many they have. Once pupils get good at this, add a question at the end of guessing how much everyone has (activity stolen from here - just under 10 minutes in).
4. Making shapes: Have a large elastic circle. Pupils step inside it and stretch it apart with their legs, trying to work out how to make different shapes (activity stolen from here).

I'm really hoping that I can pull this off. I'll let you know how it goes.