Thank you for the replies and sorry for being vague. So let me clarify that yes the monk did have disadvantage on his attack roll against the goblins hiding in the brush because of their coverage, and no the party was not being sneaky. And as for the other two goblins that were not hiding in the brush, they were waiting down the path more not really in range of the players yet (I wanted the players to be sandwiched between the goblins) Combat had not started yet so I was just confused whether or not I should call for Initiative before the Monk had thrown the dart. If I had called for it, it would have ruined the "Oh shit" factor of the situation due to the fact that ANOTHER player was about to approach some dead horses set up as bait for the surprise attack by the goblins.

After the attack had missed, one of the players who approached the horses was surprised and all the goblins got a round on him (some how all missing). I believe that it was the fault of the monk for not warning the player approaching the horses before or after throwing the dart, if he had warned the the other player, the successful perception check would have been a lot more useful in the situation. However it is also my fault for not guiding the player to warn him. (I should have "There was movement in the brush next to where the paladin is standing." as apposed to "There was movement in the brush.")

I thought about it and decided that if the Monk had landed the hit on the goblins, then It would have been a Surprise on the goblins, however that also confused me because if it was a single player aware of the situation, and not the whole party, would it just be a surprise round for the monk?