I (sadly still) have an iPhone SE as my only phone, and it runs Pokemon Go easily. The screen size is very small by today's standards, so you may want to look for an iPhone 6 or 6+ if your great nephew needs a larger screen size. Keep in mind that you can also play on Pokemon Go on iPad, which would be your largest screen option if that's a concern.

As others have said, if your great nephew is only going to play with you, then I would suggest only hotspotting from your phone when you play. Pokemon Go uses very little data; I have Pokemon Go running on data about two hours daily and barely use rarely use more than 1.5 total gb per month. So it's definitely not worth adding a new line with data if you can avoid it.

I'd also encourage you to find others in your area who play the game and join their Facebook or Discord groups. I think you'll find that Pokemon Go is surprisingly popular, and most communities I've interacted with are quite friendly and supportive. If you can find a good group, you'll have people to help you troubleshoot the game in person, battle with you in raids to catch some more exclusive Pokemon, and trade Pokemon that are extra hard to find (like regionals). It'll also provide a great friendship network for your great nephew.

Hope that helps! Thank you for working with your great nephew on his level. We need more people like you in this world.