Another alternative to Space Mountain is the Tomorrowland Transit. There is NEVER a line for it, it takes the rider through Space Mountain, and it's cool because it's the first ride to ever use that kind of mover technology (making way for rides like RnR), though it's very tame. It gives a preview of several of the rides in the area, and it's in the shade.
Also, many arrtactions have a gift shop at the end - I'm again thinking of Space Mountain (has an arcade, places to get picture stickers) and Mission:Space (has indoor benches, a cool post ride area complete with a mini-playground for the kids, and a place to send a "space" email home for free). Plus if it's really hot, there a baby station near Space Mountain, meant for nursing mothers, but others can go in to get out of the heat, too.
Also, while doing a baby swap thing is a good time to use the restroom, especially where young kids are concerned.
Another thing you can do in this situation is to split up usage of the fast passes. If you have four in your party, and two want to ride, and two don't - use all four tickets to get fastpasses (TWo for splash mountain, and two for BTMRR) usually the times will be close, especially early in the day, and you'll minimize wait time for teh non-riders.
Another alternative to the mountains in MK is for the on-riders to head over to TomSAwyer's Island. There's a playground and a snack place over there. It's closer than CBJ, and you can time it almost perfectly, as soon as the others are off the mountains, the TS Islanders can hop back on a return raft (they run continuously).
At Epcot, there are Kidcot stations - Insdide the gift shop for TT, there is a place to sit, while the kids color. Also, beyond gifts, there are a bunch of cars for the little one and mom to climb into. there is a Kidcot station inside the Land too (though I highly suggest mom does a baby swap for that one), and in each of the WS countries.