Is zakat obligatory on money saved by a father for his two children, which reaches the nisab and on which a hawl has passed, but which is constantly decreasing due to spending on them and its lack of growth or investment?

If the intention behind allocating money to the two children is to gift it to them, then it is a valid gift provided there is a witness to it. It suffices for the father to take possession of it on their behalf if they are minors. Zakat is obligatory on the minor's wealth if it reaches the nisab and a hawl (one lunar year) has passed over it, according to the majority of scholars, provided that the share of each child individually reaches the nisab and a hawl has passed. Its decrease due to expenditure does not nullify the Zakat as long as the nisab remains. The father should ensure equality between his children in gifting.