I had the same age gap between my 2 and usually co ordinated or got them complimentary gifts so they could play together if they wanted to.
I dont know if you can still get them new but my dds loved their fingerlings at that age. Hilarious for dd making it fart or burp! They did dinosaur ones too. If he likes dinosaurs though, the jurassic world super colossal t-rex or velociraptor would have the wow factor. They are huge! Not electronic but you can feed them small toys which come out of their belly. They do go down to around £25 if you watch on amazon. Also a fart gun from the minions film was a huge hit at that age, especially if they could sneak up and make someone jump with it!! DD2 is 11 and has still kept hold of all of the above 🤣
How about a doctors kit if she enjoys looking after dolls? You could get your son an equivalent role play thing that would interest him. They may even play together with those?