If you leave a gift to a child who is under 18 they will become entitled to the money when they reach 18 unless you set out another (higher) age. This is true for your biological children and for legally adopted children but not for non-adopted stepchildren.
Until the child reaches 18 or a greater age that you’ve specified the money will be looked after by the Executor. The Executors will be able to advance money to the Guardians looking after the children. When the children are old enough they will receive their share of the estate.
If you do not make a will then your children will not necessarily inherit anything. If you get married or enter a civil partnership after your children have been born then there is an assumption that your spouse or civil partner will receive everything.
If your spouse or civil partner lives longer than you and does not specify anything in their own will in favour of your children then your children may never see any inheritance.
It is therefore imperative that you make a will if you have children that you want to inherit.