In Theravada Buddhism one is usually obliged to ask for their parents' permission before ordaining. It is one's duty to respect and treat their parents with high regard. However, it is natural that not everyone may have the best relationship with their families. In that case, it can be quite challenging. One can reflect and contemplate their aversion towards the parents, and try to do small things heading their minds and healing the relationship. The teaching may be to purify one's mind by letting go of greed / hatred / delusion and accepting their parents as they are. It is the intention to do what is wholesome / in goodness and let go of the unskilful thoughts. Going slowly in this endeavour can possibly help to prevent unwholesome mind states from dominating.
Below is the sutta reference around asking Parents' for permission before ordaining
After staying at Rājagaha for as long as he liked, the Buddha set out wandering toward Kapilavatthu in the Sakyan country. When he eventually arrived, he stayed in the Banyan Tree Monastery.
In morning the Buddha robed up, took his bowl and robe, and went to Suddhodana the Sakyan’s house, where he sat down on a prepared seat. The queen, the mother of Rāhula, said to the boy, “Rāhula, this is your father. Go and ask for your inheritance.” Rāhula went to the Buddha, stood in front of him, and said, “Ascetic, your shadow is pleasant.” The Buddha got up from his seat and left, but Rāhula followed behind him, saying “Give me my inheritance, Ascetic, give me my inheritance.” The Buddha said to Venerable Sāriputta, “Well then, Sāriputta, give Rāhula the going forth.”
“But how, Sir?”
The Buddha then gave a teaching and addressed the monks:
“The going forth as a novice monk should be given by means of going for refuge three times.
And it should be done like this. First the candidate should shave off his hair and beard and put on ocher robes. He should then put his upper robe over one shoulder, pay respect at the feet of the monks, squat on his heels, and raise his joined palms. He should then be told to say this:
ʻI take refuge in the Buddha, I take refuge in the Teaching, I take refuge in the Sangha.
For the second time I take refuge in the Buddha, For the second time I take refuge in the Teaching, For the second time I take refuge in the Sangha.
For the third time I take refuge in the Buddha, For the third time I take refuge in the Teaching, For the third time I take refuge in the Sangha.’”
And Sāriputta gave Rāhula the going forth.
Soon afterwards Suddhodana approached the Buddha, bowed, sat down, and said, “Sir, may I ask for a favor?”
“Buddhas don’t grant favors, Gotama.”
“It’s allowable and blameless.”
“Well then, please say what it is.”
“When the Buddha went forth, it was very painful for me, and the same when Nanda went forth. With Rāhula, it’s even worse. Affection for a child cuts through the outer and inner skin; it cuts the flesh, the sinews, and the bones, and it reaches all the way to the bone-marrow. It would be good if the venerables didn’t give the going forth to a child without the parents’ permission.”
The Buddha then instructed, inspired, and gladdened him with a teaching, after which Suddhodana got up from his seat, bowed down, circumambulated the Buddha with his right side toward him, and left. Soon afterwards the Buddha gave a teaching and addressed the monks:
“You should not give the going forth to a child without the parents’ permission. If you do, you commit an offense of wrong conduct.”