I really think you should consider showing them the New Trilogy at some point in the sequence, my reasoning being that they should have the opportunity to watch The Rise of Skywalker in an actual movie theatre. This may sound weird, but hear me out on this. If there’s anything I missed out on in terms of Star Wars, it’s the experience of watching the movies for the first time in the cinema. Nothing can compare to watching a story unfold on a huge screen, in a gigantic room full of people, all while being bombarded by sound and music from all directions. I watched both the original trilogy and the prequels (in that order) on a tiny 4:3 television in my living room, and I remember being so disappointed that I couldn’t see them on the big screen. By the time The Force Awakens came out I was fifteen, and it had been 10 years since I saw the previous movies for the first time. Although I was excited to finally have the chance of watching a Star Wars movie in a real theatre, I was bummed out that it was happening this far into my life. I felt that it would have had a much stronger impact on me if it had been earlier.
You’ll be doing your child a huge favor by letting them watch a Star Wars movie in a theatre when they’re still young. To me this is far more important than any viewing order. A kid probably isn’t going to think about how the backstory lines up anyway, I sure didn’t when I was a five-year-old. So if this means sitting your kid through the original trilogy and the first two films of the sequel trilogy before The Rise of Skywalker hits theatres, then so be it. Just give your kid this experience while it’s still possible, now is the perfect time.