C.M.
We have the same type of kitchen table and space saver high chair. My daughter is almost 30 months old. She has actually been out of her high chair since about 2 years old. She would just rather sit on the regular chair (on her knees or bottom is what we tell her). It's best to have her sit in a chair even if you have the benches - we've found that our daughter likes to jump around the benches (which I can see the fun in it) so we put her in the chair. She's never fallen off or out. I'd try that before thinking about getting a booster.