Wills are also revoked if you divorce. Children are rather difficult beneficiaries if underage. You can't just leave everything to a 10-year old.
My parents separated when I was 9, and divorced soon after. My father's DIY will left everything to me, and was upheld without question when I was 29, but I don't know what it did or didn't say about me inheriting as a minor, as I never read it. I can't remember how old I was at the time it was made, but I do remember him saying that he had to make my uncles the executors because I wasn't old enough (he'd planned to make me executor when I came of age, but then never got round to it).
I've seen direct examples of this, it's very important. In one case the new wife inheritated everything then immediately made sure her own will excluded all of the husbands relatives, in the second case the new wife remarried and now her and and her new husband live in the family home that was originally intended to go to the dead husbands relatives.
The (2nd) husband of someone I know announced out of the blue one day that he wasn't leaving her the house because he didn't want it to go to her family when she died, so she said she was leaving him rather than wait to be made homeless. Then just as she found herself another place, he told her he didn't want her to go, and suggested moving into somewhere smaller so that she could have the house, and the balance of the cash could go to his kids.
So that's what they did, put the house up for sale, find a buyer, start selling off the surplus furniture etc, then just before they were due to move he changed his mind again and told her she could have the house. She marched him down to the solicitors the next morning and got it all put in writing.
I seem to remember that hand-written notes penned in the trenches by First World War soldiers have been upheld in law as valid wills.
My mother altered her (solicitor-made) will by scribbling all over it, but she didn't get it witnessed, so it wasn't valid.