From what I've read, Shwachman-Diamond Syndrome seems to fit what you describe, particularly due to the pancreatic insufficiency and apparent genetic origin.
I'm not a doctor, just a caregiver who reads about bone marrow-related diseases, but I don't understand why the doctors disagree. If it was me I'd be wary of doing no further testing for a year.
Bone marrow biopsies are minor surgical procedures. They can be unpleasant and involve a small risk of infection, so no doctor wants to do them unnecessarily. That's one reason a doctor may recommend only regular blood tests or observation of symptoms. If your children continue to have only mildly low counts are are living with neutropenia without actually getting sick, there's a case to be made for avoiding "unnecessary" tests as long as they stay healthy. The trouble is that the diagnosis is in doubt.
With SDS, doctors may recommend blood count checks 3 or 4 times a year and bone marrow biopsies every year or two. Before putting your children though that, I'd talk to the doctor who thinks it's SDS and ask what a biopsy would tell you, what treatments would be considered, and how the test results would affect that decision. It's safe to assume that what's wrong with one of your children is the same as what's wrong with the others, so perhaps the doctor will recommend a biopsy for just one of them, even if SDS is only strongly suspected.
Have you contacted the
Shwachman-Diamond Syndrome Foundation?