I think perhaps you may be talking about young Buddhist children who wear the robes of a monk or nun? That’s only a few of them. Well they haven’t actually taken the vows of a monk or nun. It doesn’t make sense to take a celibacy vow as a child.

They just wear the robes. I suppose it shows a kind of intention to take the vows later as they grow up but as you say, they can’t possibly truly understand the implications so it’s not a commitment of any sort.

Two initiate monks, Sakya Lamdre, Tharlam Monastery of Tibetan Buddhism, Boudha, Kathmandu, Nepal

As a famous example, the sixth Dalai Lama never took his novice vows, when the time came when he reached adulthood, and went on to write famous love poems.

Also, it is very easy to stop being a monk or a nun. You just need to say to anyone capable of understanding what you say “I hand back my robes” and that’s it done. Doesn’t have to be your preceptor. Doesn’t even have to be a Buddhist. Anyone who can understand what you say. And doesn’t matter if you have taken the novice vows or the full vows or how long you’ve been a monk or a nun for. You can still do this at any time.

So, even if the child takes their novice vows when they reach adulthood, and later on takes the full vows, if they decide not to continue as a monk or nun, they can do that at any point. They don’t need to ask anyone for permission or anything like that.