When I started the game using two self-made families (those being Marlin and Rose), later down the line caused several chains of children.
The couple from those two families, James and Daria, had two girls and a boy as triplets. Later on, Lauren (one of the girls) had two (one girl, one boy) with a guy called Daniel and Dylan (the boy) had SIX boys (first single, then triplets, then twins) with a girl called Megan. Yeesh. Talk about hard work.
Note that in all of these scenarios, the couples all met at elementary school, became partners at teens and married about 10% of the way through Young Adulthood.