updated decay children for spontaneous fission

Perhaps @katyhuff or @gidden could review this or @crbates since this is dealing directly with the decay stuff.

**Bounty** - 2
Are there additional cases that should be predicted here? If I'm understanding this correctly, it seems to me that the same problem is likely to crop up for all the nuclides that have a spontaneous fission rate.  (U238, Pu239, Pu240, Cm250, and Cf252 I think...)