Putt-putt and Freddi Fish are the best (although at 3 years you will still have to assist, at least for saving/loading the game). Pajama Sam is for a bit older kids (7-8)
My two sons started playing Loom (even finished that), Monkey Island and Maniac Mansion and the problem with all those is that they have to be able to read (this means YOU have to read), they have to learn English (meaning YOU have to translate), they’ll get stuck on difficult or even plain puzzles and nag constantly (meaning YOU will have to spoil or give hints the whole time or help them win Insult Swordfighting).
By giving them Putt Putt or something more age appropriate, which in my case I never played myself, you spare yourself a lot of effort and give your small kids a better time. In my opinion, the Lucasarts adventures are best kept for teens, not toddlers.
Another issue is that these games are so good, they’ll want to play them regardless.
That is also the reason I am currently translating Monkey Island and Maniac Mansion
So better enjoy the time while they’re young and hide away the classics for a couple of years. They will enjoy them all the better for it!
True story: I am playing King’s quest V at the moment and given its fairytale nature, I showed it to my sons. Result: I had to restart to fill them in on the story and while it is fun to think about the puzzles together, I had to replay a large part of the game again after my 5 year old was playing unsupervised and had erased the most recent savegames while trying to restore (thanks for the insta-deaths Sierra!). It will make them appreciate the rubber tree joke in Monkey Island all the better, though!