Garlic presses do not come in direct with Hametz, and thus they certainly do not need koshering before Pesah, but they should be thoroughly cleaned.
A child’s highchair needs to be cleaned very carefully before Pesah, since children often scatter food particles as they eat. The entire chair, including the straps and the tray, should be thoroughly scrubbed to ensure that all food particles are removed. Since young children’s food is not very hot, no further koshering is required for the highchair.
Kitchen counters should be koshered by pouring boiling water on them directly from the kettle in an uninterrupted stream. The counters must first be thoroughly cleaned and then dried before the hot water is poured. If a counter is slanted, one must pour from the lowest part to the highest part, because otherwise the water will drip down and the lower part will be wet when the hot water is poured on it. Every spot on the counter must have water poured directly on it from the kettle, but this does not have to occur all at once; if necessary, one may pour water over one part of the kettle and then boil more water to pour over the rest.
Although pouring boiling water suffices to kosher the counters for Pesah, many people nevertheless have the custom as an added precaution to also cover the countertops for Pesah.