Claritin for children and Benadryl (diphenhydramine) together?

Not recommended. Both Claritin and Benadryl (diphenhydramine) are anti histamines. Claritin is long acting usually used once a day and Benadryl (diphenhydramine) is shorter acting. May be used 3-4 times a day. Since they both have action, recommended not to take it same time.
Antihistamines. They are both antihistamines, so i wouldn't take them together. They both act similarly, so their effect would be too much together. Wait until one wears off before trying the other, about 24 hours or so.