Here's what we did:
We did this in our little plastic tub, to keep the mess contained.
This is a good opportunity to talk about mixing colours, too.
Things we'll change next time:
I think we'll do it without the tinfoil, but maybe make a design on a piece of tinfoil at the end (apparently, it dries on it nicely... we're still waiting for ours to dry :) The reason I wouldn't use the foil again is that the cream totally sticks to it, so the foil was lifting up all the time, with her hands when she lifted them up.
Another change I would make is not to leave the food colouring on top of the cream when she got to dig in (now her hands are dyed ;)