Are you traveling with a car seat? If so then the crew will most likely instruct you to place it in the window seat.

If you are using a CARES harness, or no type seat restraint, then I would place the child in the middle seat. That way the child is "trapped" between the parents who can take turns being responsible ( one sleeps, one entertains etc). The parents can change from window to aisle as needed.

I would not put a two year old in the aisle seat as it would be too easy for then to escape.

You might also look at reserving a bulkhead position. That way there is a wall in front of you. This can make life easier for everyone.