It's not moving past, it's moving through at 3 feet for every 1 foot
It's a bit weird to imagine, but you've basically described that there is a Small and Medium creature 'blocking' a 10' wide space.
The rules around moving through other creatures are found in Chapter 9 of the PHB under Movement and Position
You can move through a nonhostile creature's space. In contrast, you can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature's space is difficult terrain for you.
So, while each creature controls a 5' space, there are specific rules about moving through. And if the space that a creature wants to get to means you have to go into another creature's space, then the above rules apply.
In your case, you've got a medium and a small creature holding the line. As they are hostile to the winter wolves, the spaces can not be moved through unless there is a size different of two sizes.
The medium creature does not allow movement through, as they are just one size larger. But the small creature does allow movement through, as they are two sizes smaller.
Passing through the line
If the winter wolf wants to get 'through' the line, they must pass through the space of the small creature and treat the movement as difficult terrain. However, that space is too small for them, so now the squeezing rules come in. Those are found in the same section and state:
A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that's only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it's in the smaller space.
This means that the winter wolf must treat passing through the space as difficult terrain and are being squeezed. So each foot of movement actually costs 3 feet of movement (an extra foot for difficult terrain and an extra foot for squeezing through.)
Also note that if they leave the reach of either creature, the winter wolves will also trigger an opportunity attack.
A second option is in the DMG: Overrun
Chapter 9 of the DMG also covers some additional optional actions. If the table/DM decides, there is another option for moving through hostile creatures which opens it up beyond the above and ignores the size differential requirements in exchange for additional mechanics:
When a creature tries to move through a hostile creature’s space, the mover can try to force its way through by overrunning the hostile creature. As an action or a bonus action, the mover makes a Strength (Athletics) check contested by the hostile creature’s Strength (Athletics) check. The creature attempting the overrun has advantage on this check if it is larger than the hostile creature, or disadvantage if it is smaller. If the mover wins the contest, it can move through the hostile creature’s space once this turn.