I hear about it from other elementary ALTs often, but it's never been an issue for me. I play with the kids, piggy back them, play smacks junken etc but I was very clear from the beginning about my boundaries and the second anyone crosses them I stop playing and go have a coffee in the staff room. Since my attention is a sought after commodity the kid who went to far gets yelled at by the other kids, and it only takes one or two incidents for all the kids to understand what not to do. Heading into year four and never a kancho, only one boob incident.
A friend of mine has it pretty bad at one school though, the kids have devised a game called Oppai Hunter with scored for various oppai attacks and even an oppai finish. The teachers find the whole thing hilarious.
Secretly, though... I also think it is hilarious.