Skateboarding

I'd start out just trying to make it as fun as poss. Find a safe hill and have fun riding down sitting on the board.

Watch sky brown / kids skating videos. If they can see it they can be it.
Old school videos from the 80s might be good as the tricks may be simpler.

There may be lessons available locally.

Go and hang out at your local skatepark people are often very friendly helpful. Other kids may be keen to teach.

Get a board yourself and learn together or find a school friend who can buddy up.

Get yourself one too, even try a longboard, they're great fun - I wish I'd never stopped 20-odd years ago.

My daughter is a similar age & always wants a shot of the cruiser that lives in the boot of my car - like @sacredhart says, start them off sitting / kneeling to get the hang of how it moves, then stand them up on flat ground & encourage them to shimmy it from side to side to practice balancing - don't be tempted to hold on to them to help them, instead of correcting their balance they lean on you & the board shoots out from under them & everyone end up on their arse.

My wee one will be "encouraged" to get into it next year once the weather dries up a bit - good luck!

Good advice above - having a board yourself is handy (and fun!).
My daughter started rolling about when she was the same age. At first, I just spent a while with her just standing on the board, stepping on and off it and generally getting comfortable with how it felt before even trying to skate around. Then we started rolling along and she got the hang of pushing really quickly from there.

A lot of people focus on tricks and transitions and stuff, but first of all they need to just get comfortable rolling around and pushing etc - the simple stuff. Lessons won’t really be worth it until they’re comfortable at that stage I reckon.