Does anyone know the equivalency in mg between children's aspirin and low dose aspirin?

I cannot find children's aspirin at Walgreen's. The problem is I was told by a vet many years ago that I could give my cat 1/2 children's aspirin but no closer together than every 24 hours as that could cause seizures. If I give them a 1/2 of Low Dose Aspirin the MG might be higher and 1/2 might be too much.