Halflings should not be blocked to enter a blocked door for example if the door is blocked by a creature or another character that has legs that the halfling can move through.
Just how small are the halflings in your game!?
Remember that, certainly in combat, those legs are going to be moving all over the place. If I were trying to position for an arrow shot or to dodge a sword swing, I'd be less than impressed if I tripped over an impatient halfling.
To confirm with others: My halflings are "small", averaging jsut under 3 feet tall. Most other humanoids - humans, elves, half-elves, tieflings etc., are "medium" sized creatures.
In 5e, ANY creature can move through the space of a hostile creature if that creature is at elast 2 sizes larger or smaller than them (so a Dwarf (medium), can move through the spaces occupied by a Balor (Huge), treating those squeares as difficult terrain).
Halflings have a special racial trait that specifically reduces this to 1 size category - meaning that halflings specifically can move through the spaces of any medium sized creature, which is most humanoids. Moving through their space will count as difficult terrain, but they very much should be able to do it.
