Its been hinted at but no one has explained it.
There are two ways of jointly owning property. One is as "joint tenants". This is where both parties own the whole property together in "undivided" shares. If one dies, the joint owner inherits the whole of the property irrespective of the terms of the will or any intestacy.
The other way of owning a property is as tenants in common. this is where the co-owners own a share each of the property. It's normally 50/50 but can be different proportions. In this circumstance, each co-owner can bequeath his share of the property to whoever he likes.
In the situation described, the husband can bequeath his share of the property to the child if it is owned as tenants in common (although there may be things the surviving spouse can do to avoid a sale). If the property is owned as joint tenants, it matters not what his will says, the wife will inherit the property by survivorship. However, if the property IS owned as joint tenants, it is very easy to sever the joint tenancy in order to change the ownership to tic - all that is required is service of a notice.