There's been some discussion about this in the past, but I decided to get some clarification from the Hackney Unit of the Boston Police Department. In most areas of the United States kids have to use child safety seats when traveling in a car...except when they're in a taxi.
I couldn't find that same exemption in Massachusetts law. The law says that a taxi driver is exempt from the $25 fine that would usually be issued, but it never says that the kids are exempt from the law requiring them to be secured. I asked people in this forum, and some said that you don't need to use the safety seat, others weren't sure.
The response I got from the Hackney unit is that kids are NOT exempt when riding in a taxi or similar vehicle, and that the accompanying adult would be issued the citation. I can't speak to whether or not this is actually enforced, but that's what the law specifically states.
Needless to say, this is a huge headache for people traveling with toddlers. The most logical option is to use public transportation as often as possible, or rent a vehicle. My issue is that you can use public transportation, but sometimes you get stranded somewhere and it's not an option to use it on the way back. You're probably expected to just carry around the safety seat with you around town...oh well.