D.W.
We each grieve differently, and you don't have to justify your feelings to anyone.
As someone who's had cancer with 2 very small children (diagnosed the day before my son's second birthday and 10 weeks after my daughter's birth), I can say that the people in your lives help you get past the shock of the situation. Of course, our situation is much different.
My only thought is that having you there, knowing the loss you're feeling may actually be helpful to them. However, if you can't for your own reasons, I'd be honest. I'd tell them how much you want to support the family in their time of grief, but you're still dealing with your own - no one can fault you for that.
If I were in your situation, I'd send a card now - they'll be receiving a lot from other people, but the donations and generosity will die down quickly over the next few weeks. The father and children will be dealing with the loss for many, many more years. I'd probably send something for the girls randomly in a few weeks/months - put a lot of care and thought into it and let them know they're always in your thoughts and prayers.
Almost 2 years out from my diagnosis, few people bother to ask how we're doing anymore. Life moves on, people forget and think you're OK when it's a daily battle not matter how well you do putting a convincing game face on for the public.
Good luck with your mourning!