| As a technical matter, it's probably a good idea to have vitamins with or within half an hour after maybe of a meal. That should help their absorption and digestion. It can also minimize problems in case a child might be especially sensitive to any component (nausea or mild stomach irritation would be minimized or avoided). If taking them as chewables or as liquids or other forumlations which are not regular 'adult' style tablets, it's probably a good idea to make sure to brush the teeth or rinse the mouth sometime after taking the vitamins, before sleeping. The same time of day, every day is a very good strategy for any routine. That applies either to morning or evening, or whatever other time of day may be chosen. it's important when prescriptions may be required to find out from the pharmacy what interactions or changes in routine may be required because of the vitamin supplement (sometimes there can be interactions between the required routines that need to be considered and minimized) P.S. I have a guess that parents do the best they can. They will opt for a routine that works in the context of their lives and their children's lives, and their lives together as a family. A definitive study about a 'best' time of day may not have been done to assess the finer points of what may be best in an objective sense. If a child has a particular routine in place, it's probably best to work with what they know and have come to expect in this kind of thing. Morning routines can be associated with properly getting ready for one's day. Night routines can be associated with getting ready for bed and being able to sleep well. | 
