Looking after children and parents is one of the most obvious ways to gain merit
One of our teachers, Ven. Ajahn Ofer Adi, has taught us that the highest merit is one’s own practice. The question arises, then, why don’t we isolate ourselves in a cave somewhere and focus just on meditation practice? Ajahn Ofer explained it something along these lines. If you were going to climb a ladder, would you just jump up to the top rung right from the start or would you go step-by-step? Of course, we want to reach the top right away but that is sometimes too far to jump so we need rungs.
In Buddhist terms, a rung on the ladder is equivalent to merit. That is, the merit that we gain when we do good things in the world. Giving is such a simple way to gain merit and many people have an opportunity to gain immense merit by looking after their own children or by respecting and looking after their parents. Of parents, the Buddha said this*:
I tell you, monks, there are two people who are not easy to repay. Which two? Your mother & father. Even if you were to carry your mother on one shoulder & your father on the other shoulder for 100 years, and were to look after them by anointing, massaging, bathing, & rubbing their limbs, and they were to defecate & urinate right there [on your shoulders], you would not in that way pay or repay your parents. If you were to establish your mother & father in absolute sovereignty over this great earth, abounding in the seven treasures, you would not in that way pay or repay your parents. Why is that? Mother & father do much for their children. They care for them, they nourish them, they introduce them to this world.
* see Kataññu Suttas: Gratitude translated from the Pali by Thanissaro Bhikkhu