My dogs have allergies to grass..?

One is 64.4 lbs & one is 3.5 lbs. The vet told me to give them benadryl (off brand is fine). He said adult benadryl tablets for the big dog (but the off brand is a capsule, is this ok?) And the childrens for the little dog. Big dog 2 tablets 3 times a day & little dog 1/2 tsp 3 times a day. Is this even ok for my little babys? lol i am sure he knows what he is talking about but it worries me haha.