OP I'm a willwriter and this scenario comes up all the time.
The aim is always to write wills which are fair to everyone. Unfortunately an equal split in these circs, whilst it looks fair at first glance, doesn't actually achieve equality in reality if both parents die whilst the youngest child is little. That child needs financial support for a certain number of years because obviously they can't work. Therefore an equal split means that much if not all of their share is eaten up before they're of age. So their adult siblings, who have already been supported to age 18/21, then also inherit a chunk of cash in their late 20s.
That's the first inequality.
The second is that, as PPs have pointed out, the elder two are likely (although I appreciate this isn't guaranteed), to inherit from their other parent.
In these circumstances, depending on the size of the estate, number of children and the elder children's ages/financial status, I frequently advise that the clients make initial wills which would be appropriate for the time being on the basis that once the youngest child is 21, the wills are revised/rewritten.
So these initial wills should give unequal shares to the children for the reasons set out above, and then once all of them are over 21, the updated wills split the joint estate equally.
OP, you and your husband also need to consider making life interest in possession trusts. These would prevent the survivor of the two of you making a new will after the death of the first person (which usually gives everything to the survivor in the first instance), then making a new will cutting out the older children.
There's an article on how these trusts work on my website; PM me if you'd like a link.