Yes it is a kiss.
He's 5 years older than her, but he's still within the tolerable range even if it is uncomfortable. 10 and 15, while uncomfortable, is not outrageous, since they are both still children its not pedophilia. If it was 10 and 18, then it would be. It's still within the realms acceptable action between two children. At least it would have been.
Now for why its actually wrong. She didn't give consent, and its not necessarily a life saving procedure like CPR, so basically he is sexually assaulting her. And as we all know, as long as you aren't a degenerate, without permission you are sexually assaulting someone. No permission was given. It wasn't life threatening. The prince is a predator.
FYI, I can't repeat this enough, but Age of Consent in Japan is 13 years old, the lowest age among all 1st world countries. So by all standards, pedophilia is basically legal in Japan. Also, girls can get married at 16, and guys 18. Japan is a country of pedophilia. That's why high school student x middle age teacher love stories are popular there (used to be, not sure if its the case now). Yeah, different prefectures have higher ages, but Japan as a country is legally backward concerning the well being of young girls (for other things as well, but I digress). In regards to this, Japanese audience will probably find the kiss fine, even if it is the legal definition of sexual assault on a minor.