Yes, as for me the ultimate goal of Aikido is oneness with God, and a child has for example limited experience their own mortality and thus one of the major issues of existence, I am looking at "good" here from a child's point of view. "Good" in that sense refers to a child that is well on their way to building a foundation upon which they can continue to build a oneness with God.
OK, I think I understand what you're saying. I'm glad I asked, because I think that when you say "good at Aikido", most people probably think you're referring to martial skill. We could have a long debate about whether "oneness with God", by any definition, is the purpose of Aikido. My own feeling is that there is neither a singular consensus nor a universally accepted authority that could tell us authoritatively what the purpose of aikido is, and that any such debates are really taking place in the realm of "should be", if we're being honest. The practical reality is that every person who steps onto the mat brings their own purpose with them. Children, in addition, are usually at least partly driven by the purposes of others -- sometimes so much that it's hard to see what their own desires are. I don't think many children are seeking oneness with God, at least not of their own volition, but that's neither here nor there. If oneness with God is the goal of aikido, why would it matter if a child (or anyone else) quits aikido practice? Maybe they've found a better path.