We used to get Iodine in our food(bread/salt) so I don't see any problem in putting it in milk. Good for you! SMART kids! :)
I'm a niacin newbie myself, I use "Source Naturals"...start with 100 mg, that should be enough to give you the flush. Experienced niacin-ers recommend doing niacin in cycles.....what the length of time on/off that is, I dunno. Grzbear, Infinitelove & Newport are all experienced niacin-ers.... :)