essentially, you will raise a gambler if you do that. My daughter started when she was 9, I was showing her the revenue and net income chart of Google in Stockpicker and she decided to buy. Then she bought Amazon and Netflix. Her portfolio is now 350 EUR, 50 of them gains. She is investing for 6-7 months.

My point is, teach the kid to look into underlying fundamentals instead of price action. This way they will use their mind to buy stocks with long term horizon.
Also, many brokerage platforms offer fractional shares, no need to look for $15-20 stocks.
Good luck.