Something that I have done is start a new game and create sims. It can be a single sim or siblings. Play them long enough to get some money built up, get the steel bladder and then save them to the bin. Doing that several times lets me have some single male and female sims of YA that I can add to a game.
Create a family, give both parents the surrounded by family LTW, have a baby, after the baby turns to toddler, put family on static and read the toddler the nine books, teach him/her to talk, walk, potty train and then age up to child. Let him or her go to school for two or three days, then age up to teen. Have another baby, twins, do the same but now the teen can help with the babies, give a bottle, read a book, teach to walk. After the twins have aged up to child have another baby and do the same. With five offspring the LTW is fulfilled and the parents get some reward points that can be used. If you want you can age them all up to YA, os just a couple to YA, using the modify to get the traits you want. Save the family to bin, add them to a town. You can have the mother become pregnant again and have at least five new teens to YA sims with a baby to add to the town gene pool.
Couldn't be used in a dynasty, but for personal dynasties, which I play a lot of, it helps to add some decent sims who have some nice traits.
