\begin{Schunk}
\begin{Sinput}
 head(merge(x = dat_long1, y = dat_long2, by = "child_id", all.x = TRUE), 12)
\end{Sinput}
\begin{Soutput}
   child_id Time.x FSIQ Time.y Reaction
1       110      1   98      1     0.33
2       110      1   98      2     0.32
3       110      1   98      3     0.28
4       110      2  102      1     0.33
5       110      2  102      2     0.32
6       110      2  102      3     0.28
7       110      3  104      1     0.33
8       110      3  104      2     0.32
9       110      3  104      3     0.28
10      210      1   89      1     0.34
11      210      1   89      2     0.28
12      210      1   89      3     0.19
\end{Soutput}
\end{Schunk}
