I mentioned before that I GM for my three kids as well as a game here and there for some grown folks. In both situations, Unless I write my own adventure modules to play them through specifically written for small groups of PC's, 1 to 4 in my case, then I must heavily modify the game to be better balanced.
I have gotten to the point of not even running small groups of first level PC's through adventure modules. They tend to get slaughtered if I don't heavily modify the game to more appropriate for 1 to 4 PC's instead of the typical 4 to 8 that most modules are written for.
In writing level balanced adventure modules, first level PC's generally take the worst of the damage. Most of the monsters in these published modules are second level monsters or higher.
What I do now is save the adventure modules for second level PC's or higher and run all first level PC's through a series of dungeon crawls to build up their skills and levels a bit before throwing them into something bigger and tougher. Now that isn't to say that the level one dungeon crawls are a cake walk, but they stick to monsters that are 1 HD or level one monsters or less.
For example, I just ran a level one dungeon crawl last night for a couple of level 1 PC's. A newly minted Fighter and a newly created Cleric. Just the two PC's.
The first monster challenge they faced was a trio of goblins in a pitch black hallway with a torch lit.
Thanks to some really bad luck with the dice, the Cleric died and the Fighter nearly died. Three goblins, that's it.
Lucky for them, they were investigating a place just outside of a town and the local cleric was in the temple and was able to resurrect the new cleric and heal the new fighter. it cost them pretty much everything they had but they can continue on and they did manage to kill the three goblins in the course of the dungeon crawl. They earned the XP for the Goblins. The Cleric and Fighter had to take bed rest for some time, but they live to adventure another day.
The point is, life is tough for a first level PC. All it takes is a couple really unlucky dice rolls and you're toast. Whereas a series of small adventures that give several opportunities to gain XP and some bit of treasure can help get them to 2nd level and be better prepared to handle a real "full blown" adventure.