Without a doubt get the Magic Trackpad! For the adults used to using a mouse there will be learning curve but it's short. I'm in my 50's and made the change last year and since then my old mouse lives in a drawer and hash't seen the light of day in about a year. The MTP has more gestures available for it and brings out more of the power than any mouse can. Also the batteries last significantly longer, the Magic Mouse is notorious as a battery eater. Once you get over the learning curve (it took me about 2 weeks) I suspect you will feel the same as me. The learning curve is primarily made up of unlearning old bad habits of using a mouse, relearning some muscle memory that we've built up over the years using mice.
I probably would have opted for a trackpad given a choice, but since I ordered a refurb iMac, I didn't have the option. Now I'm not sure it matters. I discovered BetterTouchTool, which allows you configure many more gestures than come built in, which may have made the the trackpad moot.
One finger swipe back and forth between web pages was only possible in Safari, but BTT allows you to have the same functionality in any browser. (Chrome Canary now has one finger swipe built in).
I configured three finger swipe down to show desktop, up for Mission Control. I have a gesture to close current tab. Three finger swipe left or right (it's a little tricky to do) switches between tabs. In Mail, two fingers up and down moves to previous or next message. Three finger tap brings up App Expose. Three finger click to middle click. There are many more possibilities. It took some getting used to, but now I love the Magic Mouse.
I wanted to get a trackpad to try the gestures, but now I'm not sure it offers much that I don't already have. The only thing missing that I can think of is pinch to zoom. I might still try a trackpad, but I'm not sure it's worth the cost, or that I'll even like it. Magic Mouse with BetterTouchTools seems to offer the best of both worlds.
Back to the OP, this all works for me, but I don't have a child, so don't know how well it would work for a four year old. It takes a bit of manual dexterity to hold the mouse and wiggle the fingers in all the various configurations.