I gave my 5 year old 1 teaspoon of loratadine at 12:30pm. it is a 24 hour dose. she is now (9:30pm) having sneezing, slightly running nose. can i give her 1 tablet of diphenhydramine (4 hour dose) to get over the night?

Lower the dose: You can give the diphenhydramine if the symptoms warrant it. To be on the conservative side, if your daughter is an average sized 5 year old, you can give 6.25 mg every 4-6 hours. This is only 1/4 a regular pill (which are 25 mg each). For the liquid, it would be 2.5ml. Hope she feels better!

Yes: Antihistamines have a high margin of safety in dosing. The dose described should be fine.