You cannot have babies or children in The Sims 2 on Nintendo DS.
The DS version is a stripped-down port focused on:
Aging, genetics, pregnancy, babies, toddlers, children, teens, multiple generations, neighborhood view, build mode (walls/floors), aspiration rewards.
For the full family experience, play The Sims 2 on PC (with expansions) or The Sims 4 (free base game on PC/PS4/PS5/Xbox).