A couple options.
Adjust the module by reducing combat challenges.
Add in NPCS to bump them to a full strength party. Pets help out here too in adding combat help without spoiling plot and interaction.
My preferred one would be to bump up the PCs, so they are more powerful than a recommended PC in a full four person party would be.
This is what we did in the Banewarrens when it was me and one other PC, we had higher point buys and magic item values going in with a backstory of being higher level and then energy drained by a villain involved in the plot. I was playing a converted longtime 2e character, so we based the point buy off of his stats to make it even between the PCs and started at 7th level each (I'm not sure what level that module is for). As a 2e longtime character we had a bunch of his items destroyed in the intervening intro story, but his remaining ones still put him way over the top for a 7th level character after his energy drains (winged boots, robe of the archmagi, and sword +3).
So to bump them up there are a bunch of options
1 ECL race or template so they are the appropriate character level but more powerful.
2 More starting magic than normal pcs have which adds to character effective power.
3 higher level than asked for in the module.
4 There are also some sourcebooks with xp costs for powers without gaining levels which make characters more competent than their level would indicate.
If you want them to be a certain high level but starting characters, you might want to require them to play that level of ECL creature so they are powerful but inexperienced new characters.
