You need access to the account and all associated wallets - not just quickly being shown the screen, given screenshots or seeing conversations.
If you look at the transaction history in the wallet, this will give you a full record of where the money has gone. Money does not simply "disappear" from a binance wallet. Its possible that at least some of it still recoverable.
It is also possible that he sent it all outside binance as part of a scam, however if it was advertised within Binance, this is unlikely, as Binance generally tries to avoid people moving funds outwith their platform.
Binance is respected and regulated exchange internationally (I have my criticisms of binance, dont get me wrong, but I'm just pointing out that Binance itself is not especially scammy), however it stopped being a UK regulated crypto exchange on 22nd June 2023. Its possible that the funds in Binance have been caught in some kind of regulatory purgatory, especially if he sent the funds from HSBC which is known to be crypto unfriendly.
I guess what I am saying is that at the moment while its clear that your husband is untrustworthy, its not clear whether thats because he is incompetent or dishonest.
If its the first, its up to you to step up and do what you can to see if any of the childrens funds are recoverable, not just leave it to him and accept that if he says its gone, its gone.
If its the second, you need to face that and use that knowledge to inform any future decisions you make with him and not just quell the suspicion that he might be with a "Oh, well, there are lots of scams in crypto, boo hiss at those nasty scammers that stole our children's money, poor husband, how was he supposed to tell.".
Like I said earlier, I've been around in crypto a long time and I generally stay gender neutral on forums, so people assume I am male and I get a lot of access to "locker talk". The number of men who use crypto as a means to steal from women in various ways is phenomenal.