The expiration date is exactly that. You must enter on on or before that date. There is no difference if you arrive one, ten or 100 days before the expiration date.
Typically separately scheduled so they can prepare all the needed up front paperwork for the children but it will depend on the consulate. Your consulate instructions should indicate how they want it handled
Except that the k1 has probably 90% similarity as it relates to the consular processing except the k1 is the DS 160 instead of the DS 260. The pre interview check list and medical exam are going to be the same.
Have you done aby research? A quick Google search came back with some good information for Bolivia
https://travel.state.gov/content/travel/en/us-visas/Supplements/Supplements_by_Post/LPZ-La-Paz.html