I liked 3,1 as well but could also see 2,3. The 2,3 reasoning has been covered in this thread. For 3,1: we've had 1,1; 2,1; 1,2; 2,2; 3,1 so far. So we had every combination of two 1s (1,1) & every combo using a 2 & a 1 (2,1; 1,2; 2,2). So next up could be every combo using a 3 and a 1 (3,1; 1,3; 3,3, following the pattern set up by the 2 & 1).
I didn't come up w/3,3 on my own but that is valid too. In the 2nd column, we've had "1,2,1,2,1" so far. In the 3rd column, we've had "1,1,2,2,3" so far. In the 3rd column, it seems like the next number could be 3 (then 4,4,5,5,etc). In the first column, it could be something like "1,2,1,2,1,3,1,3,1,3" (teacher's soln) or it could be "1,2,1,2,1,2,1,2,..." (daughter's soln).
Basically a lot of these 'find the pattern' kinds of questions have more than one answer. I'm sure if you tried, you could come up with some goofy 2 or 3 (or more) term recurrence that is satisfied by the numbers we have right now, and then gets something "crazy" (like 4,7) for the next row.