I have had both. First let me begin by saying these reptiles live quite a long time- my beardie lived to about 14, my leopard gecko is pushing 20 and still alive
. The pet stores LIE about life expectancies on reptiles (especially smaller ones like geckos) because most receive such horrendous care they can only cling to life so long. Bearded dragons have an actual lifespan of around 15 years. Leopard geckos can live upward to 25 years
. I got my beardie when I was a teenager and she was already an adult, so I only had her a few years before she passed (after developing a tumor). Leo, the gecko, my parents let me get when I was 7 as my "first pet". If they had known I would be almost 27 and still have him, I'm not sure they would have necessarily let me take on that kind of long-term responsibility. I do *adore* my sweet little Leo, though he could pass any day now he's so old, decrepit and totally blind. For the past few years, I have even had to squish his insects for him and hold them right in front of his nose with the tongs because he can't see them nor move fast enough to catch one anymore. Poor old guy
.
Care-wise, I wouldn't really say either is as 'difficult' as a parrot and both of mine had/have good temperaments (could be handled). Adult bearded dragons need a FAR bigger enclosure than adult leopard geckos, so a child may not have room in their room for a 50-100 gallon type aquarium for an adult setup. Adult leopard geckos can do just fine in a 20-30 gallon setup. Yes, you can cram either in smaller enclosures, but they won't have much room to move, can become obese or develop behavioral issues where they just circle their tiny enclosure. Ideally, they both need an adequate sized enclosure with varied surfaces (rocks, wood, faux plants), a water dish, a hideaway and for beardies, a food dish as well for fruit and veg (geckos do not need fruit/veg). Both need poop and old food removed from their enclosure and fresh water daily. Leopard geckos need their enclosure to be LIGHTLY spritzed daily, especially when molting, to boost the humidity.
Leopard geckos are insectivores, meaning their diet consists entirely of various forms of insects. They also should only be fed about twice a week and may eat less in the winter. It's a hard concept to adjust to when it would be cruel to not feed most pets daily, but these guys will get obese and not live as long. Bearded dragons are omnivores, and eat insects a few times a week, baby mice 1-2 times a month (essential to their diet) along with fresh fruit and veg chop sprinkled with calcium powder daily. Insects from the store/raised at home must be gut loaded and wild caught insects (which both mine got all summer long since they are very nutritious to insect eating reptiles) MUST be caught in areas where no pesticides are sprayed (some people adamantly oppose feeding wild insects). For a child, it may be difficult to have to feed a beardie a baby mouse once or twice a month, but they are very nutritious for them and a part of their diet you shouldn't skip.
Bearded dragons MUST have a UV bulb, and while many say leopard geckos don't need one, mine has always had one and in fact comes out during the day to bask sometimes. And despite the claims, leopard geckos are not sand dwellers and SHOULD NOT be put on sand (even the calcium based repti sand)! Leo almost died as a result of a sand impaction (every time he ate a bug, he got a mouthful of sand too, effectively cementing his insides together), and has been on reptile carpet since. It was a $400 vet bill (back in the 90's, so I'm certain it would be more costly today!) since my parents couldn't let my first pet die on me less than a year after I got him! It took 6 weeks of my dad having to give him nightly baths in warm water and syringe feeding him mineral oil and some kind of liquid nutrition to keep him alive and try to get the impaction out since I was too young and my mom was kind of scared to touch him. The exotics vet who treated him said it's not uncommon for leopard geckos to die of sand impactions. Beardies are sand dwellers though and are designed by nature to live on sand.
Other factors to consider: Leopard geckos are mostly nocturnal, meaning they are often sleeping when we are awake making for a slightly boring pet to a child, and no those little hideaways where you can see them through the glass aren't ok for the reptile just so a child can see them sleeping. Leopard geckos like a nice, safe, dark hideaway to sleep the day away in in their enclosure. Bearded dragons are definitely sun lovers and are active during the day. Both shed their ENTIRE skin regularly and cannot be bothered at all when shedding. It is always fascinating to watch them turn white then tear off and eat their old skin! Both are receptive to being hand tame if worked with properly, though both have teeth and can bite (though I doubt either could break skin). Leo likes being held because hands are warm, Tiffany's favorite place to be in the whole world was curled up around the back of my neck. As soon as I'd open her enclosure, she would bolt up my shirt and assume her rightful place! Both can also be 'trained' to tolerate a repti-harness so they can go outside
A lot of people assume reptiles are 'boring', but they are absolutely fascinating IMO and can be quite friendly, learn to recognize their owner and enjoy mental stimulation. Let us know if you get your son one!