If taken with other metabolites that aid in the creation of serotonin - vitamin B, niacin and magnesium, which a multivitamin should provide - 5HTP can help deal with many of the same health issues that people take tryptophan for.
If all you take is one over-the-counter multivitamin, you're probably okay, but if you start mixing your own cocktail of vitamins and minerals, you have to pay attention to your total daily intake of each thing.
Popping a multivitamin will of course never take the place of a wholesome, home-cooked meal, but if the option is to live off McNuggets and Hot Pockets exclusively anyway, a supplement is better than nothing.
The multivitamin will contain what you need of all the "energy vitamins", and taking in 100 times the RDI only means you'll flush out the expensive surplus next time you go to the bathroom.
This involves following the American Dietetic Association (ADA) nutritional guidelines for children, and possibly the addition of a multivitamin if the pediatrician feels it is advisable.