Non-assessment application - How do you do it in practice?

I have a children's account with ING for each of my three kids. I then invested everything that came in over the years from grandparents etc. in various ETFs. The oldest (12) now has an impressive 25k in “credit”. So that everything doesn't get eaten up by taxes, I submitted the application and submitted it to the bank. How do you usually proceed now? Do you sell everything once a year and buy it again straight away? Or how do you do that? What do I have to pay attention to? Thank you submitted by /u/