The people of Thedas are too busy enjoying their disposable income I guess. Dragon Age never seems to have a whole lot of children. While DA:O certainly had the most, it only has 4 total, and only two of them are named NPC's you can interact with.
Edit: correction, 5 children with the little girl in the Stone Prisoner DLC.
Nonsense, there are way more kids in DAO.
Origin stories:
You've got three kids in the City Elf Origin, who can all be talked to and all will reply.
Couslands origin has three kids that can be spoken to and one wall climber. (During the army of cloned soldiers and servant scene).
Mage origin has 3 children who all have lines, although they can not be talked to.
Dwarves don't have children. They spawn from rocks carried by Deepstalkers through the Ortan Thaig.
Dalish elf has four children loafing about (wanting to be you and missing Tamlen).
Total: 14 little demon spawn.
Main story:
Lothering has 8 in camp, 4 before bridge 2 after , 2 in the tavern and 3 in the chantry. (total: 19 (maybe +1 because the little ships kept wandering around.))
Haven has 1 little brat.
Denerim has six kids running in the market (all birthed by Goldanna), then there is the puppy child and the sitting child. The Alienage has Amethyne, two caged kids, one dead child. (12)
Redcliffe has four in the chantry and one in a closet. (Don't remember if more spawn after the siege.) (5)
Dalish camp has seven kiddies. (7)
Then there is Amalia in Honneleath.
The circle has three living ones. (And probably a festering pile of dead ones as well. Those pink throbbing piles of flesh on the wall? Children.). (3)
There might be more but the guards are starting to look at me funny.
So about 46 kids in the main game, around 60 in total in the game. (Only counting one cut scene kid, because he's too creepy to be forgotten. )
About 7 of the main game children have dialog related to quests. 3 of the Origin quests have actual dialog and almost all of them have general dialog lines.
Dragon age 2
There are two children in dark town in every act. And of course the Urchin.
Act 1 has Sabina's brat, the boy Anders revives and Pryce if you sided with Athenril.
Act 2 has Ella, but I'm guessing she's in her late teens.
Act 3 has Cricket and Walter.
So there are about 6 or 7 kids in DA2.
As for why are there no children in DAI? (Besides Morrigans kid) They probably didn't bother to make the models or figured that DAI was too much of a danger zone for them.
But to be honest I'm not really sad that the children are missing in DAI. There are plenty of those noise makers living near my house don't need them in the games as well.