Alright. I think i misread things and thought you were after an (strictly) educational kit. Just wanted to say that. We gifted an (quite comprehensive) electronics kit to a seven year old and that seems to be a good age to start. but under 5 i can't see a kid having the attention span, dexterity or mental abilities to grasp concepts aside from on/off, this is a light and this is a switch. but i may be wrong. there is certainly no harm in starting too early. i just think it wont be fun or of value for a 3 or 4 year old. in my experience they get bored quickly if you try and convey theoretical concepts. at that age i see kids playing with wooden tools, train their dexterity with a small hammer and nails game. or mimick their parents and play something like cooking or doing the dishes, that has something to do with their every day life. nonetheless. try it. i'll bookmark this and read all the ideas and experiences of other people. maybe i'm completely wrong. one thing i observed kids are interested in all kinds of silly stuff. and they start asking questions as soon as they can. and i believe it is a good thing to encourage them in asking questions and figuring out concepts and how things (including physics) work.

edit: some dads build a big wooden box with (old) sturdy buttons, switches, indicator lamps, a vandalism-proof keypad, etc for their toddlers. i saw a few blog posts years ago. But that was completely DIY. I don't think it teaches anything but dexterity, but toddlers like pressing buttons. And it's a cool project. And a few years later you can use it as the main console for your imaginary space ship. ( https://youtube.com/watch?v=j6zseFi070E )