I played a game against a 6 years old child where I launched a strong attack against the poor child:

[fen ""]
[white "Me"]
[Black "6 years old child"]

1. e4 c5 2. Nf3 e6 3. d4 cxd4 4.Nxd4 Be7 5.c4 Qc7 6.Be2 a6 7.O-O Nf6 8.f3 Bc5 9.Be3 Qb6 10.Nc3 Nc6 11. Na4 Qa7(11...Bxd4 12.Nxb6 Bxe3+ 13.Kh1 Bxb6)  12. Nxc5 Qxc5 13. Nf5 Qf8 14. Nd6+ Kd8 15. Bb6+ Ke7 16.Bc5 Qg8 17.Nxc8+ Ke8 18.Nd6+ Kd8 19. Bb6+ Ke7 20. Nxb7 Qb8 21.Bc5+ Ke8 22. Nd6+ Ke7 23. f4 Kf8 24. e5 Ne8 25.Nf5+ Kg8 26.Ne7+ Nxe7 27.Bxe7 Qc7 28.Rf3 g6 29.Rd3 Ra7 30.Qa4 Qb6+ 31.c5 Qxb2 32.Re1 Qb7 33.Bf3 Qc7 34.Red1 Kg7 35.Rxd7 Qb8 36.Rd8 Qb5 37.Qxb5 axb5 38.Bf6+ Nxf6 39.exf6+ Kxf6 40.Rxh8 Rc7 41.c6 Ke7 42.Rhd8 Kf6 43.R1d7 Rxc6 44.Bxc6 h5 45.Rf8 Kf5 46.Rdxf7+ Kg4 47. Rg7 Kh4 48.g3+ Kg4 49.Rxg6+ Kh3 50.Bg2++

He played strange moves in a few seconds in the opening, e.g. as 5...Qc7 or more concretely 11...Qa7 missing the exchange of 3 pieces for a queen (which Stockfish says is good for black) much better than the position where black finishes after 13. Nf5. He also moved his bishop twice in the opening, his queen twice... all this with his clock over 1 hour and 30 minutes in a standard 90'+30'' game.

Once he lost a piece he spent some minutes to play his king's moves after the checks with only two or three possible moves. It seems that he thought more about it when defending himself, but when I checkmated him his clock was on 1 hour 12 minutes for a 90'+30'' game.

Why do children move so quickly in standard games, specially in the openings?

In the tournament I am playing half of the players are children and after one hour and a half all children have finished their games and there are only adults playing.

I assume once a child joins a chess tournament it means he or she doesn't find it boring to play a chess game. I am just asking why they don't spend minutes in tricky moves as adults do. Why don't they use their time?