(1) They don't allow much experimentation. 'Follow these instructions' and 'This kit is so small you can't do anything other than follow the instructions anyway, unless you buy ten of them.'
(2) They are not age-appropriate. I'm probably not the only one who ignores the 'This kit is intended for kids aged 7+' warning on things that a 2-year-old is fully capable of doing on his own. I am also not impressed by kits which require children to exert more force to build something together than the kids have at their age, or toys where all mechanical and electrical parts are so well hidden that you can't watch them work.
3. They don't explain natural phenomena. It's cool to see a homemade rocket take off, but why does it work? Simply building without the accompanying explanation and theory doesn't make the most out of the experimental learning experience.
And how to react?
Seriously, what do you give to a toddler who isn't even in preschool and wants to build a wind turbine and differential gear? My son showed us how to 'get experiment kits right' – creativity is the key. He's taken everyday household objects and put them together in rather creative ways, having built his own rattle from a set of cubes and nuts, a valve by inverting a cup into another of slightly smaller size such that the water doesn't fall out, and even a model rotary engine out of a tape holder, champagne cork, and flower pot (all without parental supervision or involvement – we only got to see the results). I need to go and drill some holes in beer caps ('cog wheels') on the weekend, maybe I can convince him to give me back the salad strainer he's been trying to connect spokes to in order to make a differential gear… in the meantime, we're growing crystals, playing with supervised small explosions, and all sorts of other safe things kids enjoy.
Whatever happened to kids’ chemistry sets?
The first chemistry sets for children included things as dangerous as uranium dust and sodium cyanide. Now they’re safer and a bit less fun.