Traveling alone for work, I only have a carry-on, which means that if the driver tries to help that only slows things down. For that I don't tip.

With my 2 1/2 year old daughter (+ car seat, stroller, extra suitcase, usually my wife's suitcase too)....I move my butt to get the bags in & out of the shuttle as fast as possible, and if the driver helps (which they almost always do) then I make sure to tip $5-10, say thank you, and have my daughter say thank you too.