We've done both. And enjoyed both a great deal.
My kids are a bit older, 7 and 5, but they have great fun doing the watermice and the campfire program.
Sorry for my laziness, but I'm going to copy and paste a post I did on the DVC board about the watermice:
A water mouse is a little two person speedboat. It rides low to the water and generally is a lot of fun.
There are restrictions....I believe you have to be 60 inches tall and 12 yrs old to DRIVE them. A child younger or shorter can ride with an adult. Two adults can ride together, but there is about a 320 pound maximum. Speed is based on the weight in the boat, so if you have wto people totalling close to 320 pounds in that boat, you aren't going to get the speed that you'd get if both got their own boats. Clear as mud?
They run about $30 for a 1/2 hour (don't quote me on the price) and there is a DVC discount. There is also a length of stay plan that costs $199 and is good for water mice, pontoon boats, canopy boats and bikes (not surreys though) The plan is good for up to a family of 5. We just used it this month and had 4 in our family and 3 guests and they signed up the 5 adults on the plan. The kids could ride free in watermice and other boats, but when we did bikes we had to rent separate bikes for the kids because they were not listed on the plan. Still a GREAT deal. We ended up (between the 5 of us) with about 6 hours of pontoon boat rentals and about (rounding here) 12 hours of watermouse rentals and about 2 hours of bike rentals.
IMHO, the best place to get the watermice are somewhere on Bay Lake (WL, CR, or FW) or somewhere on Seven Seas Lagoon (Poly or GF) That way you have access via the water bridge to both of those huge bodies of water. You can really have some fun jumping the wakes of the bigger boats and can open it up for long periods of time. I'd suspect that you couldn't get the speed or have the same experience at OKW or DD just because of the smaller sizes of those bodies of water. We've never rented them at those places, so maybe someone else has a different experience.
GrammieMame gave you the general low-down on the campfire program. I can only add a couple of things....get there early!!! Not only for a good seat (the benches were comfortable, haven't ever sat in the bleachers) but because it becomes a zoo around the fire as it gets closer to movie time. We showed up at 7:15 both times for an 8:00 movie. At that time they were just starting the fire and there were just a few kids around. Later there were 30 or more kids around some with flaming marshmallows!
Another fun thing to do with kids is go over to the barns at Fort Wilderness. It's where all the parade horses live. They also have a petting zoo and pony rides.
Hope this helps!!!