I'm an austistic adult who is hyperlexic so I just want to chime in here to say that hyperlexic kids very commonly turn into quite academically burnout and exhausted adults. Please look out for that if you suspect your kids fall into that category. Please be mindful of celebrating them without unintentionally pushing their special interests, intelligence or academic achievements as their personality wholly, or what makes them "them" if you will ...it's a hard identity shift to unravel in adulthood. Also it's quite often diagnosed alongside autism, it was a major part of my autism being diagnosed as a child. I flew under the radar a lot due to being academically focused and articulate from a very young age. While some of this may sound positive please just be aware that it can actually be quite an all encompassing hyperfixation that distracts from other developmental or social milestones, that inherently internalises a connect between self esteem and productivity.
Other than that, any specifically themed encyclopedia, interesting looking thesaurus' or games centred around words have always been a favorite item of mine to be gifted, and very much still are. Do with that whatever you will lol.