We had this situation OP. In Laws started by saying they wanted to give money towards DS's university costs. When he was 16 we discovered that they had been investing money for him with their own financial advisor, and that when he turned 18 he would have £100k plus of his own!
We were pretty horrified, we didn't really know the details of what they were planning and would certainly have advised against this way of doing it.
Funnily enough, there is no way on gods earth that they would ever have given DH, their own son, the equivalent amount of money! I think that the tax efficiency implications were also more important to them than the thought of what an 18 yo might do with that amount of dosh.
By the time my DS turned 18 FIL had died, but I think that MIL suddenly realised that they might have made quite a bad mistake.
He is 20 now, and all is well so far. He is at university, we pay tuition fees and he pays all his other expenses and so we have avoided a student loan. He is a sensible boy, he has met the financial manager who handled the investments for him. We have all agreed a reasonable amount for him to access each year. He is not wildly extravagant, but he was able to afford the uni ski trip without any extra debt, and I am pleased for him for that.
However, there is a downside as well. He has never had to work for extras, and like others I do worry about what could happen if he fell into bad company. There is absolutely nothing we can do to stop him if he wants to blow the lot.