I know this is an old thread, but there's lots of reasons it would be good for your kids to know this.
First, it is equality in action - sexuality is about much more than sexual attraction. Pretending bisexuality (or homosexuality or heterosexuality or pansexuality etc) are just things that matter in the bedroom is nonsense. It denies people's lived experiences, and it denies the ways in which who we are attracted to shapes how we live our lives. Your kids deserve to know that sexuality is not some secret to be kept in the bedroom - either because they themselves might end up with a non-straight sexuality, or because they will be adults in the world and them understanding realities around sexuality would be good.
Second, there are plenty situations where your DH's bisexuality might be expressed in ways that the kids will be aware of. It might be if you're watching TV and you might comment on someone being quite attractive, or perhaps there will be a time your kids ask you about your first kiss or first BF etc. Or if your DH has experience of coming out, or experiencing biphobia or hate crimes etc, these may be things he sometimes just mentions in casual conversation (after all, almost all of us sometimes talk about events that have shaped our lives, and for many LGBT+ people, coming out or experiencing discrimination are part of that). It would be wrong to deprive your DCs of a relationship with their father where they are not allowed to fully know who he is and what his path in life has been.
And third, and sorry to get grim, but even though you are committed for life, if your marriage ever ended or (sorry to be morbid) you passed away and your DH went on to form a new relationship - how awful would it be if the first time your kids found out about DH being bi was if he went on to form a relationship (or just go on dates) with another man. How shut out of your real lives might they feel knowing that you and he knew this but kept it from them.
Drop it in early and in a low key way, before your kids learn to attach undue weight to disclosures about sexuality. At some point, maybe when they're five or six, I would contrive to have a bit of a jokey conversation about mummy's past boyfriends, and Daddy's past girlfriends and boyfriends. Don't do it as some major disclosure - just drop it into convo as the most normal thing in the world. They'll probably pick up on it and ask some questions. That's your opportunity to say that some people like men and women, and so before daddy met mummy, he had boyfriends as well as girlfriends.
At that age they won't care much, and then they'll continue to grow up with that knowledge as something that was just always part of their knowledge - no big bang disclosures that might cause them to feel they've been lied to or that they never really knew their dad.