Thanks, Amy!
1. While a SMALL kid (to me they are ALL small...) stands in front of you with skates on both you and the kid, push on the kid's shoulders with enough force to cause motion. BE CAREFUL, small children sometimes don't need lots of force to fly across the room! Note, the kid moves backward a much larger distance than you. Keep in mind, I am 250-ish. Most of my kids are in the mid 100's. This works for me all the time. If you are on the small-ish side, pick a LARGE kid and YOU move, not him. Same effect, different direction.
2. Do this in reverse IF the kid and you (or two kids, if you choose.) are OK skaters. Head toward each other with enough speed to actually notice a change. It doesn't have to be blazing speed, but fast enough to note a change in speed when you collide. Grab onto each other upon contact. Note, if the mass difference is large enough and the speeds were the same, the "group" now moves in the direction of the larger person. The little guy got "gobbled up".
3. HOVERCRAFT - I use a home-made hovercraft, see my hovercraft page, to demo 1st Law. This is absolutely GREAT due to the "lack" of friction and it's actually quite dramatic!