I can think of 2 ways:
A) You buy for them a different ticker (e.g. VWRL, or iShares alternative; or what I would do - VT as I am buying VTI+VEA+VWO for myself), then it’s clear what ‘belongs’ to whom
B) Just account for it in an Excel sheet
And if you feel nice, also account for the received dividends throughout these years. 
Same here too, I buy on a bi-monthly basis for ~CHF 200 of VUSA at DEGIRO. So just pick one ETF which you think makes sense and use invest exclusively in it from your “child” money. As bonus here, pick one ETF which is in the DEGIRO list of free ETFs without fees 
Definitely the best option speaking from experience on the other extreme. I’d do that if I had the option.
I would just ensure that I gift the same amount to both when I decide to do it. Gifting to optimised taxes and gifting it directly into an investment account to get them on the right track 
Been looking a bit: nice advantage for Migros is that it can be transferred to the minor at 12 y.o. already and not 18. so less risk for heavy bank oversight and get the money “released” to my kids 6 years earlier .
I may go this route but I’m not inspired by any of their funds…. Problem still for my daughter though that is younger.
Decided that I’m gonna try this way. BCN has worn me down. Better to get access like this than wait 10 more years…thanks for the advice. Probably won’t even invest it as I try to transfer it out asap. Let’s see.
Cheers! 
MP faced the same issue some time ago
In his post he mention that he was able to withdraw the money from the bank under certain conditions
The banker explained to me: Then we need a valid written explanation that you are going to use your children’s money to buy something that is for them (with proof of purchase ideally)."
As mentioned in IB: Monthly Account Inactivity Fees Eliminated - Investing / Brokers - Mustachian Post Community, with the monthly inactivity fees being eliminated, it is now super easy to set up an account for each of your children there.
Thanks but I think in his case it was several hundred (max 1000chf). I have 20k for each child so the bank said I could take it out with proof EVERY time…it would just take me 10-15 letters to do it that way. I think Migros will be easier in the year.
Small Update…took me a while to make time to do it, but finally went in to speak directly with MigrosBank and PF about transferring the accounts in my kids names from the bank I set them up in (who won’t let me invest them anymore): Migros will let me transfer the money to my own account and get back control of this money to invest myself on behalf of the kids. PF was very imprecise so not pursuing.
I just did that last week. Ready within 2d. Free of charge and nicely separated.
For those who want to invest with accounts in the kids names, I can vouch for the youth fund savings plan from CASH bank zweiplus. So far it’s the best option I’ve found, and I use it for my kids personal investment funds. The kids enjoy knowing that the money is in their name, and I think it’s a good lesson in investment. But it’s still not as favorable as investing directly in an ETF on their behalf.
Good ideas here, useful comments, especally @JMH, @Mobius .
I have been accumulating some stocks for my kids but on my own brokerage account (Cornertrader). Now I had an account setup for my son in his name (I have no access, he is 18) but want to transfer the stocks to him. I understand that there is no tax under 50’000chf (I live in Vaud), no problem, and that he’ll need to include the gift in his tax statement. but:
how do I document the gift? do I / him need to create any document that will be used for tax purposes and … any other? have you tried such move?
A signed letter and transaction record from broker should be sufficient.
Now I don’t know how to ask broker. Who is your broker?
Well, you will have to instruct your broker to transfer to someone else (not yourself), won’t you?
Sorry, I can’t help. I’ve no experience there. I would expect that if you’re less than the allowed ceiling for gifts, they won’t give you too much hassle since it will just come off your bottom line. Your son will probably need the documentation more than you, so wouldn’t hurt to even call the tax office to be sure of no bad surprises.
In your tax return there is a section for gifts given/received, you fill in -50000 to son, he fills in +50000 from father. Since your wealth has (probably) decreased by 50k and his has increased by 50k at the same time, no documentation is needed, as all very transparent.
Thanks @rolandinho , that helps. I was worried that some kind of document needs to be drawn. I’m still trying to read on Direct federal, cantonal, and communal taxes: How much do you have to pay?, but no more info (apart from tax free gift limits).
and yes @San_Francisco , this will have to go through the broker, should not be a problem
To close my loop: happy to say that Banque Migros respected their word and I had access to the money as soon as my e-banking was up. So have access again on everything saved for my kids and feeling pretty good about investing it for them for the next 10 + years!
Thanks for the advice @sugoi worked exactly as you mentioned! Really appreciated!!
is there a easy summary of the thread?
I started a degiro account in my name and i plan on buying VT with it. Is it still the “best” play?
Without bringing which broker, US/Ireland etc into the discussion which is a whole other thread, I think so. At least, that’s what I’m doing too. 