This is child friendly, save for the obvious stove/pouring a lot of hot water part. Letting the pot of water sit to cool off is just fine, so long as the tea is brewing in it. Don't worry about straining tea out, since this is not drinkable once finished.
IMPORTANT NOTE! Make sure you discard the tea once you're finished, and do not let it sit out longer than 2 weeks or it will start to smell ...weird. It's definitely an odd smell. I do not recommend forgetting this exists.
Things you'll want:
1. Something to pour tea into. I have a large concrete mixing bin because I'll sometimes soak full sheets of watercolor paper. Get a flat container that'll fit the size you'd think you'll work most in. The bag in this I reuse (I hose down when done). It's what our bed came rolled up in and the plastic is REALLY thick.
2. Things for texture. Plastic bags, leaves (I used mint, strawberry, and dandelion in this mix). Sticks are cool. Old crumpled up paper works too.
3. Tea! The stuff I used was just generic tea bought on clearance. The boxes were bought probably 5 years ago, it's been a while since I made tea art. Different teas stain differently. Green tea is VERY light. Fruit tea might look neat (reddish sometimes), it will not stay red on the paper. If I remember right, darjeeling will stain pretty dark.
Feel free to remove paper after a day, let dry, toss back in again for another layer. I've left paper to stain for a week + in time.
4. Pot to boil water.
5. Watercolor paper. I prefer 300lb cold press, but I don't have any on hand. Printer paper will fall apart in your fingers. You'll want something a little thicker.
6. Somewhere flat to dry and flatten paper with once finished.
Let me know if you have any questions! I've been doing this since college (I want to say 2000 was the class I first started doing this in).