I think a "Lego" sort of product at a low price point with lots of available components is very much needed. I'm not sure about another "Grove" type of product. Really once you want to go beyond what's available already in the "Grove" space you might as well just use solderless breadboards and jumpers.
I see these "Lego" type products pop up from time to time but none of them seem like they'd allow the average kid to really be able to dig in deep without spending a ton of money. LittleBits was getting a lot of hype for a while but the kits really only allowed you to build a few different configurations so I think most kids would quickly get bored with it. The additional parts were very expensive ("wire module" for $7!!!) so it wasn't really feasible to get creative with it.
I think for you to make something better and cheaper than what exists will only be possible if you can do mass manufacturing, but mass manufacturing is only possible if you either have a lot of investment or a hugely successful crowdfunding campaign.
I'd recommend you to closely study the other products in this space, both the ones that made it to market as well as the ones that didn't. I already mentioned LittleBits, another is LightUp. Arduino even had a try at a "Grove" type product, the ESLOV but their crowdfunding campaign didn't go well.