MY current family consist of 22 people. Mother, Father and 20 children. Mother and Father are Fully grown adults. 4 of the children were born at the hospital. 16 were from time traveling, they were 9 elders, 1 young child, and 6 teens. So how do you get a time machine child?
First you need to complete the inventing skill. Make a time machines and place it in your house. Then you need a young adult/adult/elder male sim and a young adult/adult female sim. Click on the time machine and choose to woohoo with the two chosen sims. If you want a child/teen woohoo in the past. If you want an elder child woohoo in the future. You have to have room in your house for the child to join your house hold. (maximum of 8 people per house hold). You also may need to wait a day. If after 24 hours you havent gotten your time traveling sim child try again!

By the way there is another way to get a child which is pot luck. Time traveling to the past or future your sim may interfier on something happenin while they time travel and may bring a child back with them!

Enjoy your simming!