It depends on what your relationship is to the children and how well you know them and their mother.
My husband died when our children were aged 8 and 3. They are now 14 and 10. They have both rejected all offers of counselling/play therapy etc, and most of the time don't want to talk about his death or their feelings with anyone other than me. They find it intrusive and upsetting. (I also don't like it when people seem to want to be impromptu counsellors.)
However, the children do like it when other people who knew their father talk about him - not about his death or their feelings, but anecdotes and memories about things he did when he was alive. It is important that they don't feel he is a taboo subject. They also liked it when other people gave us photographs (eg one former colleague of DH's put together a photobook for us).
I think unless you are very, very close to these children, or are a trained grief counsellor (which I presume you are not, or you wouldn't be asking the question), you should not try to 'help' them - just be a friend to the family, mention their father in passing when appropriate, and talk about him if they seem to want to, but don't keep bringing the subject up deliberately.
Practical help for their mother might be appreciated, perhaps offering to babysit so she can get some time to herself, or taking the children out for an outing somewhere.