I was going to get frustrated with you and say "do not have official gym leaders or Elite Four members to travel around with a random kid because that'll make him more of a Gary Stu", but then I noticed "Kid and Kid". You didn't specify who the other kid is, so I'm going to take it he or she (most likely a she, which makes this cliché, to be honest with you) will be an original character. And thus, I'm more lenient towards that decision than a canon character, especially those of importance. Everyone else pretty much said what is needed to be said, but I'll also say this: be wary of the relationship. Chances are very high you're going to have a small romance between Ken and the girl travel buddy if you are going with a female character. While I'm not against it, you have to make sure you develop the relationship the best you can without getting to the point immediately.
I'm being reminded of the ending of the climax scene from Secret of Nimh 2 where Timmy runs back inside the burning laboratory for his brother, and his friend, who is a girl he met some time ago, shouts out to him "I love you!" and he goes "Love you too, whoo!" There were really no instances in the movie that showed the two had enough time together to really form some kind of romance. She really was just there to be the love interest that was so obvious it would happen. If that's what's going to happen, then I'm going to tell you "no partners" (especially since Ken has his talking Pikachu to keep him company), unless you can prove to me you can develop their relationship reasonably. You need to prove your worth, RetardRaichu, that you can give equal development to these two characters without butchering them or making them complete Mary Sues--which is an issue you're having at the moment.