At the start create a young adult, an elderly person and a toddler.Style them.

Go to done, then press X to change the relationship. Make the young adult and the toddler Mum and child, then click finsh.

Then go back into create mode and make the young adult into a teen. Start playing.

There you have it - a teen with a toddler for a child.

PS: if you go back into relationship mode once the young adult is a teen then the cheat will no longer work.