Reservations never hurt, but like previous posters have stated, you can get seated in a reasonable amount of time as a walk up, depending on the time of year and the time of day.
Another thing that helps is to have a Rainforest Safari Club card. You are given priority seating on arrival, guaranteed reservations (based on availability), discounts (10% off 4 entrees) and special email offers. We have had one for several years and it has really been great when we take our DS to Rainforest Cafe or T-rex. You can also use it at some of the other Landry's locations. Landry's also has a Landry's Select Club. I don't know how this works at T-rex, the web site says you get priority seating points that accumulate and rewards from those points. We also have this card but have never used it at T-rex. Unfortunately, I think you have to go to either a Rainforest cafe for the Safari Club or other Landry's locations for the Landry's Select Club, I don't think you can get either card over the internet. If you are near a Rainforest Cafe, I would imagine it would make for a great outing with the children, they almost always seem to love the place.