Have you looked into Burning Wheel?
It has a much more detailed melee combat system, where every player and GM character gets multiple actions per exchange (3-5 usually).
Mouse Guard simplifies the BW Fight! mechanics pretty significantly, to make it more accessible to a presumably younger audience.
The thing to remember is that individual mice (even mice with swords) are lunch for most everyone in the wilderness. This is why they team up. Teaming up means team planning (choosing actions in advance and then acting on them in turn), as well as team risk (One disposition for the whole group). Mouse Guard RPG is very much an all-for-one one-for-all sort of system, especially when dealing with conflicts. This is to the benefit of the mice, because 3 mice working together have a much better chance of getting the successes needed to fight off a Milk Snake (throwing 8 dice) than a single mouse does (Lieam notwithstanding). Each mouse can play to his own strengths in an action (with help from the other PCs), and then let the next mouse do his own thing on his turn.
I ran a four-mouse team against the Raven from Delivering the Mail and they did much better as a single team than they would have been with two teams each with two fewer dice (and, on average 1 fewer success) each roll.
(oops… there’s a whole extra page of exchange there. I went and added my two cents when others had already done a much better job than I. Oh well. )
I will take a very different approach from the other responses you have had until now and hopefully give you something more tangible in the “game world” then it’s a group of mouseguard and such because as I see it that’s what your players are struggling at.
When another player is “helping” in a conflict, they are not only facilitating the action of the performer but may also be seen as doing the same thing.
I.e: If it is a Fighting conflict and you choose the Attack action, all your mice may effectively be seen as striking with their sword. The thing is if that snake is performing a maneuver, it effectively affects all those trying to attack it at once… It is not attack vs attack but attack versus another “action type”. Thinking this way it is not farfetched to visualize things based on the rule system.
One attack action is not necessarily just one strike it can be seen as multiple strikes by even the performer, so what are a few more blows from the other helpers, nothing much then 1D really and if the other party is defending by squeezing between 2 rocks, it affects everybody at once.
In my opinion, you need to widen your visualization of the conflict scenes and see them in a more strategic way then in a tactical way.
Hope this helps; it put my players to rest anyways
To give an example of “helping in action”
Nola is taking the action. It’s a Attack. Folker and Saxon are helping.
Steph (Nola): I’m running under the fox, aiming for it’s testicles.
Jerry (Folker): While she does that, since I’m alread at it’s tail, I’ll swipe at the hamstrings. Steph, help die.
Steve (Saxon): Well, I’ll take another poke at its’ whiskers to keep it distracted as Nola makes the run. Wil, that good enough for the help die?
Wil (GM): Sure is. Steph, you’ve got the help die.
With groups, it’s the same idea. Give an one liner for what you’re doing to justify the help die.