Let x and y be the price of a senior ticket and a child ticket, respectively.
Thus, the two equations mentioned in the question are
[tex]\begin{cases}3x+y=40 \\ 3x+2y=52\end{cases}[/tex]Subtract both equations as shown below,
[tex]\begin{gathered} (3x+2y)-(3x+y)=52-40 \\ \Rightarrow y=12 \end{gathered}[/tex]Thus, the price of a child ticket is 12 dollars.