Op, none of these is either a bad idea. The both of them are brilliant and a very great move to take as a young man planning future for his children. However, there is nothing that doesn't has it negative and positive side of it, everything in life definitely has, it all depends on our personal choice as humans. In my own personal opinion, I prefer investing in housing especially since it's what I intend leaving them for my kids when I'm no more. There is no specific reason for saying so but I just believe it will be more easier for them to manage when I am not there to guide them than Bitcoin investment. Please nobody should quote me wrong. What I am saying is that for the fear of if they might be scam due to their negligence or ignorant in it and maybe they might fall victim and lost everything along the line.
It actually depends on the interest or priority of the father or even the child
Some father's don't mind asking the child what he wants, either Bitcoin portfolio or houses? At this point, the child gets to think and decide which is needed or preferred.
In a case where the child is too small and the father cannot ask such questions, then I think its just beat the father decides which will be more favourable for the child. He knows the child and should have a little clue on how technical he is and what he might possibly like or handle. But if am asked to choose which is better, I would prefer Bitcoin investment, because even while they are grown, housing wouldn't be an issue since they can still leave in there father's house while they figuir out how long they can get there bitcoins or get ready to use it.