My ten year old has seasonal allergies for the first time. I gave her 24 hr Claritin (loratadine) at 7:30 am. It's 9pm. Can I give her children's diphenhydramine?

Okay. This is fine for short or longer term treatment. You might consider some of the newly made OTC nasal sprays with a low dose steroid (Nasocort,Rhinocort,etc.)I find many young women don't like the sprays, but if they will give them a week or two the allergies get better and they can sometimes drop the pills.
Yes. 10 year old with seasonal allergies, got claritin this morning. Yes, it is safe to give her children's benadryl (diphenhydramine) before bedtime. However, benadryl (diphenhydramine) only lasts a few hours and causes drowsiness, should not be given on a regular basis. There are other treatments for allergies. Get her checked out by pediatrician to make sure her symptoms are consistent with allergies, and for more treatment options.