The midfielder was in the Under-23 side which beat Sunderland 5-0 at St James's Park last night.
Ben Dawson's side dominated from start to finish, and Luke Charman (2), Owen Bailey, Callum Roberts and Elias Sorensen were on target in front of a 3,000 crowd.
Longstaff's younger sibling Matty, also a midfielder, came off the bench in the Premier League 2 Division Two game.
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"I think it's every young lad's dream to get to play against Sunderland at St James's Park. To beat them 5-0 ... I don't think anyone was expecting it," said Longstaff, who impressed in pre-season.
"We were really relentless, took our chances, and I think we were pretty comfortable. I don't think they troubled us a lot.
"It was a great game and all the lads are buzzing. All in all, it was a great night.
"Hopefully, for the lads looking to go out on loan, that shows the level that we're at. It gives our lads the chance to get their name out there and show what we can do away from Newcastle."
On playing alongside his brother, Longstaff told BBC Radio Newcastle: "I think we needed about 25 tickets. It's great to share a night like this with your brother is unbelievable. It's what you dream of as little kids playing in the garden. It's a dream. Hopefully, he can kick on now and we can start games in the future."
Longstaff was called into Rafa Benitez's squad for the club's pre-season campaign.
The 20-year-old – who spent last season on loan at League One Blackpool – played in all but one of the club's five friendlies.
"It's been amazing," said Longstaff. "If someone had said to be at the end of last season you're going to do what I've done, I would have said 'you're not the sharpest', or something like that.
"To be able to go away with them twice was great and to be able to play in Ireland was great. Playing against Porto ... they were in the quarter-finals of the Champions League last year and I was in League One. That's a bit of a difference.
"I've really enjoyed it. I'm trying to get my head down every day and work on little weaknesses. If I can improve them, there's hopefully another opportunity down the road."
Longstaff is likely to be involved in Newcastle's Carabao Cup tie against Nottingham Forest at the City Ground on August 29.
"I had one eye on the draw," said Longstaff. "I'm not looking too far ahead, I'm taking it day by day. If I get a chance in that game, I'll do my best and see what I can do."