Each character can be surprised individually
The rules for Surprise states:
The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn't notice a threat is surprised at the start of the encounter.
Roll Dexterity (Stealth) for each of the PCs, and for Klarg, the Wolf, and Goblins.
For each PC, compare their Passive Perception to the value of the Dexterity (Stealth) check for each enemy. If a PC doesn't notice any enemies, then they are Surprised.
For each enemy, do the same, compare their Passive Perception to each PC's Dexterity (Stealth) check. If that enemy doesn't notice any PCs, then they are Surprised.
What is Passive Perception?
Passive Perception is a characters general awareness. It is calculated as follows:
10 + all modifiers that normally apply to the check
If the character has advantage on the check, add 5. For disadvantage, subtract 5. The game refers to a passive check total as a score.
For example, if a 1st-level character has a Wisdom of 15 and proficiency in Perception, he or she has a passive Wisdom (Perception) score of 14.
Consider using a group check
Instead of rolling Dexterity (Stealth) for each character, perhaps consider making a Group Check for each side:
When a number of individuals are trying to accomplish something as a group, the DM might ask for a group ability check. In such a situation, the characters who are skilled at a particular task help cover those who aren't.
To make a group ability check, everyone in the group makes the ability check. If at least half the group succeeds, the whole group succeeds.
Instead of rolling for each individual character, make 1 group Dexterity (Stealth) check for each side, and make 1 group Passive Perception check for each side. Then compare the values and decide if either side are surprised.
There is more info in the Hiding section
If you want to know more about how Stealth functions, check out the Hiding section.
Keep in mind the environment
Where and when the ambush takes place may affect how easy it is to hide or spot others. You may want to award Disadvantage or Advantage depending on how favorable the conditions are. Hiding in the dark, in shadows, or in fog can all make someone harder to spot with sight:
A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.
A heavily obscured area--such as darkness, opaque fog, or dense foliage--blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.
Keep in mind that locating a hidden enemy isn't a purely sight check. You could also locate them by hearing their armour clink, listening for footsteps, or smell them.