No way! I played hide and seek with kids on trains all the time. More like peek-a-boo than hide and seek.... Once the parent noticed what was going on, all I did was say, "kawaii" and they melt. I have no idea whether it's because nobody seems to notice one another or people are too into themselves to engage others or what, but, the smiles and appreciation I got from the parent was second best to the smiles I got from the kids....
I wouldn't go out of my way to approach them or make any kind of physical contact, but if you're on a train or in line and a child is near you, try to smile at them and see what happens....
Just so you know, not that many Japanese carry a katana with them all the time.... just a few here and there....
