Restaurant high chairs are notorious for having broken safety restraints and other defects. They can be dirty too, since chances are the chair you plop your toddler into hasn't been cleaned since the last child used it.
One option: Bring your own portable hook-on chair (keep one in the trunk of your car). The JPMA currently certifies four brands of hook-on chairs: The Graco Travel Lite Table Chair, which retails for around $34.99, Chicco's Caddy Hook-On high chair ($30), the Dorel Eddie Bauer Portable Hook-On Chair, ($39.99), and the Munchkin Feeding Friend Portable Hook-On Chair ($42.99). Consumer Reports has not tested them, however. Use one with a table top only, not with tablecloths or place mats.
If you do use the restaurant's high chair, clean it with a baby wipe before putting your child in it. Clean his home high chair after every meal, too.