I have not done that at Disney, but I have been a waitress. Most tables are have an even number of seating (unless up against a wall, then you lose an end seat). Most likely you are at a table for 6, moving you to an 8 top could be a challenge. Youd have better lusj if you were an odd number to begin with, then most likely youd have an extra chair. Being 6 already and getting a seat for 7 would completely move your table. Since groups of 8 or more have their own dining like and need to reserve as 8+, the larger tables most likely would be full. Is the 3 year old too large for a high chair? That may be the only way.