Until the end of middle school, I think cash is best. Keep them away from anything virtual as long as you can. I still believe a $1 bank note remaining in your wallet/envelope says more on what you can spend than some random number on an app. Couple this with a monthly allowance (ours increase with age) and discussions about how to save, how to plan for spending, etc.

Also: if you want to teach your kids how to budget, have a budget yourself and spend money like you want them to spend money. They observe what you (parents) do and learn from them, consciously or unconsciously ...

For our 6th grader, we bought a new Nokia 3300 and got him ting at $6/month with minutes included (no data). If he calls/texts more than $10/month, it's deducted from his monthly allowance (another $10/month). He went to $12 once, never did it again

20 minutes of procastination before getting up?

https://www.youtube.com/watch?v=Lp7E973zozc
How to stop screwing yourself over | Mel Robbins | TEDxSF

Hi!
Late 30s here (both) and although we use and post on Facebook regularly, we don't post pictures where the face of our kids can be recognized. We've heard and read so many stories of facial recognition being used by governments and private firms and stories of pictures on social media being used against people later in their lives. We don't go to these extremes but we trust our kids' judgement when they will be adult to say / do what they want on social media for themselves (rather than imposed by their parents, years ago).
YMMV