I want them to be happy and I want to be there for them but I dont know whether to try and help or leave them to it!
Leave them to it. They choose their behavior, then feelings will ensue. You cannot MAKE people happy. They arrive there on their own.
Care for your own emotional health.
It's fine to be supportive in practical ways -- walk the dog, babysit the kid, bring dinner and leave, whatever -- so they don't have to tend these chores so they have a block of time that is free of other obligations while they sort themselves out.
It's another thing to have them both airing their stuff at you and you feeling anxious and "stuck in the middle." That's not healthy sounding to me. Encourage them each to find someone ELSE to talk to about it. Counselor if need be.
Let them sort it out on the (BF <--> Partner) tier of the polyship on their own. When they have news you will receive it from the (BF + Partner) <--> You
tier of the polyship.
I know you feel strain in the
(You <--> BF)
(You <--> Partner meta)
(You + BF) <--> Partner meta
mini relationships in the polyship because when one person in the polymath
feels out of sorts it is felt all over. Like if someone flings themselves in the waterbed, all other people sitting on it go bobbling about.
But keep clean boundaries. You can be supportive and helpful in practical ways, but in emotional ways don't let yourself be put upon or put in the middle. That's not healthy for YOU.