[RPG] Would giving the halfling player characters darkvision significantly unbalance the game

It does not cause any issues. My group does max HP as our default setup.

The only difference is removing an element of random from the table which most people are in favor for when it comes to mundane but important elements like HP. It removes the "flavor" added by being bound by the dice constraints so it is unlikely that 2 lvl3 fighters with the same CON mod have the same HP.

Just don't forget its class hit die PLUS constitution mod PLUS favored class (if applicable) each level up

Yes, it is

By level 30 there are all sorts of epic tier powers available to players to regen health, have massive heals, avoid taking damage at all, or basically avoid death once per day. All of these powers are in there to compensate for the deadliness of the creatures that players will face at that point.

Contrariwise Different classes (generally by role) have different HP gains per level to reflect the nature of both their class and role. Defenders get a lot of HP per level (and often have a higher con as well) because they need it, its part of their roles. Likewise, the rest of the roles get less (striker down to controller) because the nature of the roles makes them less and less apt to be the focus of damage. There are of course some classes that tend to have more HP for a given role, such as the Warlord, because their class is more likely to be found on the front lines.

The Fix

Simply recalculate everyone's HP to where it should be by the rules. 4e is a very, very balanced system with a few exceptions (some dragon magazine content that never got erratta'd or a few classes that never quite worked out) and it's in your favor to keep things RAW vs. introducing house-rules that can cause unintended consequences rippling through the system.