Can I visit at night with kids? We won’t get there until 8:30pm

From the visitors guide "The Village and Desert View on the South Rim are open all year and park entrances remain open 24 hours a day. North Rim facilities open mid-May and close mid- October. Park entrances remain open 24 hours a day during this time. Hours for visitor centers and businesses vary throughout the year."

Yes

The Village Loop inside the park on the south rim has lodges and restaurants however you won't be able to see any of the canyon after the sun goes down it's pretty dark.