By not allowing a child the option to quit how can that teach self discipline? It is one thing to not allow them to quit for the sake of boredom or that they feel it is "too hard" and another to not allow them to quit. It is one thing to tell them that they need to work through hardship and another to not allow them to quit. One teaches self-discipline and the other teaches them that they have no power to control their lives.
Without the self self-discipline becomes discipline. I would rather not have Aikido become punishment. Aikido may not always be a joy but it shouldn't become a punishment. Aikido shouldn't become a struggle of power between a parent and a child.
Children have lots of places that they have no power to control their lives - it does not teach them to be powerless. That's a modern fallacy. Moreover, there are lots of areas, including areas wherein a child's desire is governed and ran through their guardian's sense of wisdom. Again, this does not ruin children.
Another way of looking at this list, is to go ask someone that you think is good at Aikido - your sensei, for example: Ask them if they have always loved Aikido training, if they never went against their whims and fancy in order to continue training, if everything they ever did and/or accomplished in their Aikiido was done so only at their full and complete volition, etc. See what they say. Or, if you know a master of another trade/art, ask them.
Or, another way, if ask yourself how good little Jonny is going to be when they only commit two hours a week to training, cancel some of those classes during soccer season, decided they like baseball better, and video games, see no reason to train in Aikido outside of techniques on the mat, and then quit.
Sure, you might be able to say he has self-discipline, he's self-empowered, etc., but you won't be saying he's skilled at Aikido. Along the same lines, no matter how many Jonnies we know, one cannot by extension say that any child that sees his/her way through the rigors of sincere training, unlike Jonny, can in no way be self-disciplined, self-empowered, etc.