\begin{Schunk}
\begin{Sinput}
 merge(x = dat_long1, y = dat_long2, by = c("child_id", "Time"), all.x = TRUE)
\end{Sinput}
\begin{Soutput}
  child_id Time FSIQ Reaction
1      110    1   98     0.33
2      110    2  102     0.32
3      110    3  104     0.28
4      210    1   89     0.34
5      210    2   91     0.28
6      210    3   95     0.19
\end{Soutput}
\end{Schunk}
