You might call your vet to get the proper dose. Remember that children's medications are usually for children over 2 years old, and they weigh a lot more than 15 pounds. It will probably be just a portion of the children's dose.
Can I Give My 15 Pound Jack Russell Children's Benadryl For Seasonal Scratching?
Vet told me 8-20-09 to give 1 mg. Per lb. Of Benadryl for licking feet and scratching with no signs of fleas. Also can give twice a day safely for 2-3 weeks. I use the liquid and also the solid tablets are 25 mg. Each so we give 3/4 of tablet for our 21 lb.