They gave the cashier $20 x 2 = $<<20*2=40>>40.
Since a $1 change was returned, this means the tickets cost $40 - $1 = $<<40-1=39>>39.
Two adult tickets cost $9 x 2 = $<<9*2=18>>18.
So, $39 - $18 = $<<39-18=21>>21 was spent for the children's tickets.
Each child's ticket costs $9 - $2 = $<<9-2=7>>7.
Therefore, there are $21/$7 = <<21/7=3>>3 children.
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