That's what it sounds like. If she really only gave him a small portion of an adult dose, I'd say there was no intent.
Anti-histamines are considered one of the safest drugs. It is likely she didn't think it would harm him.
I had a 115 lb yellow lab that ate about 6 to 10 12-hour dimetapps. After the people at the animal poison center quit laughing, they told me either the dog would sleep it off or would be hyper, but not to worry.
“I had a 115 lb yellow lab that ate about 6 to 10 12-hour dimetapps.”
My mother-in-law, who was a nurse and really should have known better, once gave her cat (how much does a cat weigh? 10-12 lbs maybe?) 2 of the phenobarbitals that her late husband had taken after his brain surgery (to prevent seizures I think). He was a very big guy 6’6”, he must have weighed at least 250, and he would take 1/2 of one pill a day.
Why did she give her cat these drugs? Because she was having the carpets cleaned and wanted the cat to sleep.
Sleep she did, for about 3 days in a drug-induced coma, but then she was fine.
I still wonder about this, my m-i-l was usually a pretty sensible person.