Or not fly at all on commercial aircraft....if a 3 YO kicking a seat could harm your back, then moderate-to-heavy turbulence or a rough landing could cause far more harm. Or the olympic gold medal winner of the decathlon could be on your flight and trip and shake your seat. Or an old man could need to hold onto your seat back to get up and down from his seat to go to the bathroom every 10 minutes. All are pretty equivalent to a rambunctious 3 year old kicking a seat. If you are really in this much jeopardy of an injury, then its time to look into other means of transportation.
On the train, you have a few bumps and sways, but nothing that should injure a person sitting in seat.
Anyway, I have two small children and we've addresses this problem by sitting 2 by 2, one row in front of the other. We put the kicker behind his big sister. On jetBlue, I've also paid-up for the even more space rows.
I suggest bribing a family with small kids to sit in front of you, if it's just the two of you. Some small coloring book or souvenir and a kid won't mind being occasionally bumped.
And sometimes you just have to ignore kid-haters on airplanes. That means preventing/doing everything you reasonably can to keep the kid from kicking/annoying other passengers, but also ignoring those who cannot be satisfied. Some people get annoyed at every little peep they can hear from a small child, all the while having a full volume conversation with other passengers in her row, so that 3 rows forward an backward of them hear every detail of her feelings about religion, politics, and how to raise children.