I might have been the most excited person in my family when we brought tadpoles home -- a childhood dream realized! If you have a pond nearby, go get some! It is fun and easy to care for them. The plant nursery down the road from us has a pond teeming with tadpoles every spring. When we buy our annual plantings we bring a mason jar to bring home some tadpoles too. Tadpoles like to bask in warm, shallow water so they are easy to spot and catch. If you don't have a local source, there are frog kits you can buy. However, they seem to mostly be pet frogs. Be aware of the species of frog if you are going to eventually release them in the wild.
This is also a great opportunity for the kids to learn about life cycles (in a way that's tangible and applied to real life, which is always more fun). Here's a worksheet you can print, color and create a chart of the frog lifecycle.
Taking care of tadpoles is simple. Get the kids to set up a little habitat of rocks in a glass or plastic tank. Don't let them get carried away if it's glass (we realized this just in time). If you have well water, just fill up and you're good to go. If you have unfiltered city or town water, use bottled water or let your water sit out for 24 hours (to let the chlorine dissipate). We changed out the water every few days (when it starts to stink, basically). We would just leave a little bit for the tadpoles and then fill up the tank again (instead of catching them and then changing the water in the tank).
We fed them aquatic frog food that we bought at the pet store: a little pinch every day for 10-15 tadpoles. You'll get a sense for how much they eat. Try not to overfeed because you'll have to change the water more often. Pro tip: if you wait to catch them when most of them have little legs sprouting, you'll have teeny frogs in a few weeks. The first time we did this we got the very first tadpoles and did not have frogs for about 6 weeks.
Water plants (from a local water source or pet store)