I'm sorry for your loss. Judging by what you've said, I would guess impaction. Any loose substrate is very dangerous for baby bearded dragons. He wasn't eating enough at all, and the crickets were too large. It's possible that he was sick before you got him. Did you ever do a fecal test? He may have had parasites.. again, sorry for your loss. If you try again, make sure to do a bunch of research first. You could post a thread asking for everything you need to know. An older dragon would be easier to care for - they're sturdier and less fragile. My thoughts are with you.
I wouldn't beat yourselves up too much on this. If he/she was only eating 1 to 2 crickets a day that is not normal at all. A healthy baby dragon has quite the appetite so it's very likely you baby wasn't healthy to start with. Also 1/4" crickets aren't really a problem for a 5" dragon but improper temps would prevent proper digestion. (Not saying your temps were wrong, I don't know)
I really don't think it was impaction, rather a sick baby to start with.
As mentioned by other posters, if/when you decide to get another, the older the better. You don't have to go with an adult as it's fun to watch them grow, but 2 to 3 months is a good age to start with.
This is kind of off subject.. but I buy my crickets from the bait store all the time. They sell salamanders and lizards occ. and buy them from dealers online made for feeders. I'm friends with the owners of the bait shop and they know I buy them for feeders and have told me they feed them to the animals they sell. They also feed them gut loading cube things for that reason. I'm just saying check with the owners, some privately owned places might be like the one I go to.
I agree , the beardies at petco here look sick to me. My advice is to ALWAYS buy from a breeder, no matter what age you decide to get. Im new to beardies and I bought a 7 week old baby and with the advice and all the research i done BEFORE hand, she is thriving beautifully. If you got it from a pet store ,I would take it back. demand satisfaction. alot of pet stores dont take care of the animals properly and frankly , it makes me sick. If your gonna work in a pet store , learn about the animals that are there , it should be a requirment if you ask me.
Im so sorry for your loss.I would be devastated if anything ever happened to Kiku.