Yes, you can leave your estate to whoever you choose. Children tend to be a popular choice and they can still inherit, even if they are under 18.
Where a child is under 18 their inheritance will be held in a trust for them until they are 18. When you make your will you appoint the trustees to oversee the trust.
Until the child reaches 18, the trustees are able to use the inheritance and any income from investing it for the benefit of the child.