The fact that they would need someone telling them if a relationship is good for them or not kind of points out that they aren't mature and old enough to make good decisions on their own and probably shouldn't be having relationships with such an imbalance of power.

I don't think children need to be told these things; I think they need to be listened to when they say they receive enjoyment and pleasure from a certain type of relationship or activity, rather than having some adult with power over them making the decision for them based on a combination of sentiment and ageist assumptions. I recommend you read the Rind Report and the results it found.

The reason a power imbalance exists in the first place--or at least why it's as pronounced as it seems--between at least older children (say, six and up) and adults is because of the legal situation children presently find themselves in, and the way our entire society has every interest in keeping them servile to the gerontocentric dictates of our culture. Further, it's virtually impossible to have any type of relationship without some degree of power imbalance between the two people in the relationship; men used to always have more power than women did, a rich person can be said to have more power than a non-affluent spouse who is economically dependent upon them, etc., et al. We only see power imbalance as a problem if it has to do with age, and younger people are well known to be capable of manipulating older people to their advantage.