Your 18-month-old will be fine on TSM. I've seen newborns go on it (although I'm not sure why the parents would do this).
Just put your child in your lap rather than in a seat of his/her own that way you have more control over him/her. It will mean that you don't get to play (which isn't cool for a lot of parents because they want to play, so they put the kid in the seat next to them), but so long as he is facing forward and under the lap bar, it will be fine.