As a preschooler, I absolutely adored Lemmings. Still do, but I was unable to play it for a few years -- which seemed like a very long time back then -- because we switched from DOS to Windows and it took a while before we found the appropriate port.
When I was (probably) 7, my entire family except for Mom fell in love with a now-thoroughly-obscure (apparently it got good reviews at the time but sold poorly) TPS RPG adventure-game thingy (I suck at classification) by the name of Outcast. I was terrible at it and needed help all the time, but it was fun, and it provided more family in-jokes than any game before or since.
I don't count most of the edutainment games I used to play (although I did play a lot of Magic School Bus and Math/Reading Blaster games), but the original Zoombinis will always have a special place in my heart. I loved that game when I was in second grade, missed it for years when they took it off the school computers, and got more enjoyment out of it than I should really admit to when I finally got the chance to buy it for myself six years later. I was always a fan of the Oregon Trail, too, although I played... I think it was the fourth edition, rather than the original.