The Dhamma says that not helping parents is evil and preaches to take care of the elderly parents. What can be some positive outcomes the children will get by helping the parents?
The Buddha says that there are two people that you can never repay, no matter what you do. They are your mother and your father. Again the Buddha said that there is one way that you can repay for all what they have done. That is to establish them in this Dhamma Path. It is not an easy task, as we are all full of defilements that muddy the waters and prevent us from seeing this path. So if they’re stingy for example, then you can try to find some way to influence them to be more generous. If they are not the type to observe the five precepts, then you could try to get them to be more virtuous, to have more principles in their lives. You can gain merits that will help pave the way to your own Dhamma path, if you could help introduce your parents to the practice of Dhamma. Try to do it in as diplomatic a way as possible. Most parents resent their children who try to tell them what is right or wrong. That is why you will have to find some means of doing this in an indirect way. If they are not into Dhamma, try not to say that this is a Buddhist thing. That is how SN Goenka thought Vipassana Meditation to the Westerners. Only later that they get to know that this is the Noble Eightfold Path that their Master is leading them to.
Helping parents engages the mind in the moral 'right view' of knowing what has been received as benefaction (gift; sacrifice) from one's parents & one's resultant reciprocal obligations (good actions to be performed). This is called 'katannukatavedi' ('gratitude'), namely, 'what others do for me; what I must do in return'. The Pali scriptures state:
And what is the right view...siding with goodness...‘There is gift, offerings, sacrifice [from] ...mother & father. There are fruits & results of good & bad actions. There is this world & the other worlds. There are spontaneously born beings (sattā opapātikā)...’
Having this 'right view' of 'katannukatavedi' also helps develop skillful means in the other relationships in life & can also help develop the higher Noble Eightfold Path because 'giving up' (vossagga) selfishness is a primary factor for developing meditation & the Noble Eightfold Path.
When having such right view, the mind spontaneously arises in the 'human' (wise) or 'humane' (compassionate) state.
If parents are not helped, when required, the mind is spontaneously born in the 'animal kingdom' (of ignorance & immorality) & probably eventually in 'hell' (suffering) due to wrong view. All people age, start dying & experience loss and those with wrong view generally are not able to cope with their own inevitable aloneness, old age & imminent death.
The above said, sometimes parents can be very stubborn, even to the point of death, therefore we should not always expect we can help our parents, as we would like.