Well, with 6 kids or more in each family taking care of small children was a full-time job of its own. Not to mention not having things such as a washing machine...
With 6 or more kids in each family older children frequently helped with chores and helped take care of the younger ones. Even a 5-year old is capable of helping with simple childcare tasks and a responsible 10-year old is capable of being a babysitter for periods of time.
Children would also be naturally spaced because breastfeeding around the clock typically continued until the child was at least 18 months old, meaning that they would normally be 15-30 months apart (the amount of time that breastfeeding acts a contraceptive varies widely from woman to woman but typically breastfeeding on demand will halt a woman's reproductive activity for at least 6 months).