My assumption with children in D&D is that if I hit, kick, stab, or otherwise do even a single point of damage to them, they go 'POP' and are no longer relevant to any discussion at hand.
Hence, no need for stats.
What's the AC of a child?
I kid, a little, but basically I hear you saying, "For various reasons, I don't want to have children as a part of the stories I tell." I can empathize with that, regardless of what your reasons are, but I come at that from a different direction.
I once considered starting a campaign with a device similar to that used by Stephen King in It, where the first adventure was the backstory for the main adventure - the PC's adventures together as children. For that, I not only needed detailed stats for children, but a detailed way of deriving the stats of the PC's child selves from what the players made as their adult self.
As I said before, I've been in a long running campaign were the PC's started having children, getting married, and so forth. Once you've go 5 or 6 in game years, its quite possible to have a your PC's children as reoccuring NPCs in the game. Knowing their stats is amusing.
In my current campaign, one of the early missions involved rescueing a group of kidnapped children and keeping them alive. The difficulty of doing this would be raised to the near impossible level if children went 'Pop' after taking but a single point of damage. And as a point of realism, terrible as it is to think about children being injured, children are surprisingly durable, sometimes surviving without serious injury things that would kill or cripple an adult.
One of my better miniscenarios (IMHO) involves rescuing a child that falls into a an ancient well - that turns out to be a sacrificial cenote.
In an earlier campaign, I had a gothic horror scenario that involved a pack of half-vampire children.
In another I had the players wipe out a tribe of hobgoblin bandits, only to find in the back of their cave a knot of their children. (Those were statted to go 'Pop', because the thing I was interesting in exploring was genocide.)
I've had on the back burner a side scenario for a while now involving a child sorcerer born with the evil eye and the players involvement with his family and with the townsfolk when the kids existance is discovered. Likewise, I've had on the back burner a side scenario where a child adopts a slaad as a pet and nurses it to health in her basement. While neither NPC is meant to be a combat challenge, neither is exactly a defenseless weakling compared to at least the low level PC's I'd present this challenge to. Knowing exactly what each is capable of is I think as important as knowing the abilities of any other NPC that might directly threaten the players. I wouldn't necessarily want the players to kill either NPC, but on the other hand I can't rule that out as a choice.
And other children have had various minor roles. A world without children is incomplete and a bit too clean for my taste. I don't feel you have much of a claim to 'gritty' if you don't have children. They are useful for conveying extremes of all sorts of things - innocence, hope, courage, depravity, etc.
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