The hereditary component is very slight I believe. The Myeloma Introductory booklet states:
..people who have a close relative (parent, sibling, child) with myeloma are up to twice as likely to get myeloma than the general population
The Myeloma Info pack states:
There is a slight tendency for myeloma to occur in families. Although rare, this suggests there are inherited genetic factors in myeloma. However, these alone are not enough to cause myeloma but may make an individual have a slightly higher risk of developing myeloma – other environmental factors also need to have an impact before it develops.
Although close relatives are twice as likely the overall risk is small anyway so although it sounds bad it amounts to a small risk. I am sure the Myeloma Info line would help you talk it through. I think the most important thing is that children are aware of it so that later on, if they are feeling unwell they can ask the GP to do a Myeloma blood test.
Good luck with it all.
Paula