My 8 year old son is having seasonal allergies. I am giving him childrens Claritin but was wondering if he can also have childrens Benadryl (diphenhydramine)?

Other better options. Even certain "non-sedating" antihistamines like zyrtec have been found to impair short term memory and have negative effects on school performance. Rather than increase the likelihood of your 8 year old son having side effects, consider adding a topical nasal steroid spray like Flonase or a complimentary medication like singulair (montelukast). Alternatively, consider allergy testing to identify the triggers.
Probably fine. Every kids a bit unique with meds, but the joy of Claritin (loratadine) is it doesn't cross into the brain & cause sx. Benedry on the other hand does & can cause dose related side effects.Most people get drousy & some get hyper. Either way I have not had problems with this combination when mothers have used it before. Some patients with breakthru on Claritin (loratadine) do better on Allegra & some are having irritant rxns.
Why. If Claritin is not sufficient to control symptoms, it can be changed to another long acting antihistamine otc. Benadryl (diphenhydramine) is great for short term use with severe symptoms (hives) but it has side effects that bother most. After playing outside, have him shower and flush his nose with saline to get the pollen off. Keep windows closed. See his doc to ask about other medicines that might help if needed.