I know parents are on Platform 9 3/4 when the Hogwarts Express leaves on September 1st. Do they also wait on the platform when the train returns with students for holidays?
No, they wait in King's Cross station on the Muggle side of the barrier
He, Ron, and Hermione passed through the gateway together.
“There he is, Mom, there he is, look!”
It was Ginny Weasley, Ron’s younger sister, but she wasn’t pointing at Ron.
“Harry Potter!” she squealed. “Look, Mom! I can see —”
“Be quiet, Ginny, and it’s rude to point.”
Mrs. Weasley smiled down at them.
“Busy year?” she said.
“Very,” said Harry. “Thanks for the fudge and the sweater, Mrs. Weasley.”
“Oh, it was nothing, dear.”
“Ready, are you?”
It was Uncle Vernon, still purple-faced, still mustached, still looking furious at the nerve of Harry, carrying an owl in a cage in a station full of ordinary people. Behind him stood Aunt Petunia and Dudley, looking terrified at the very sight of Harry.
“You must be Harry’s family!” said Mrs. Weasley.
Harry Potter and the Philosopher's Stone - Chapter 17
Harry read and reread the letter from Sirius all the way back into King’s Cross station. It was still clutched tightly in his hand as he, Ron, and Hermione stepped back through the barrier of platform nine and three-quarters. Harry spotted Uncle Vernon at once. He was standing a good distance from Mr. and Mrs. Weasley, eyeing them suspiciously, and when Mrs. Weasley hugged Harry in greeting, his worst suspicions about them seemed confirmed.
Harry Potter and the Prisoner of Azkaban - Chapter 22
Uncle Vernon was waiting beyond the barrier. Mrs. Weasley was close by him. She hugged Harry very tightly when she saw him and whispered in his ear, “I think Dumbledore will let you come to us later in the summer. Keep in touch, Harry.”
Harry Potter and the Goblet of Fire - Chapter 37
There is also a much longer passage at the end of the fifth book which shows the Dursley, Weasley, and Granger families all waiting on the Muggle side of the barrier.
I would think so. How would students get to their homes otherwise? Imagine taking a bus with an owl and two trunks while wearing a cloak.