Miniman's answer is the best - per the rules of the game, multiattack can't be used within the readied action mechanic.
The one element I would add to the answer, though, is don't let the rules get in the way of your game. You're the DM and you say what goes. The books/rules are here to make the fantasy world more consistent, which makes it feel more real, but ultimately the rules submit to the game - the game does not submit to the rules.
In your specific case, when I envision the scenario, I think the distance makes it something of a stretch... I see some spears fly from the bushes which the characters react to, and then they're ready for whatever would pop out, so I'd lean toward not letting the bullywog leap from the reeds and still follow up with a bite - by the time the frog-man's feet hit the ground, the characters are attuned to the added danger. But that said, if they were much closer and in melee range, I 100% would give the bullywogs their multiattack, as (in my mind's eye) that seems much more fluid and quick... you can get off both a stab and a bite within a second or two before your opposition really comprehends what just happened to them.
Underlying point being: D&D (and RP games in general) are meant to make the entire world come to life and enable the players to do quite literally anything they want. There obviously cannot be rules that encompass "anything they want", and so the rules we do have are going to be imperfect and generalized. As the DM, you get to decide when those rules apply, and when they do not, and though I would have made a different judgement call in your specific scenario, I think you were well within your rights as "master of the game" to allow it to play out the way you did.