Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
Now you have not listed a weight for Baby Girl, but a quarter of a 25mg Benadryl tablet is a top dose for cats weighing around 13kg cat. If she is under a year, then I do suspect you have given too much. Now it is not uncommon to see drooling/foaming in cats with this medication due to its bitter taste. And that will usually settle, but it can also cause nausea, vomiting, diarrhea, and sedation. Therefore, if she has had this much, then this could leave her unsettled and why we are seeing hiding/reactive behavior.
In this case, there is no antidote, so we need to just monitor and let her settle over the next 5-8 hours. Water should be kept available, but we'd need to rest her stomach. As long as she doesn't appear overly sedate nor starts vomiting profusely, then you can continue to monitor until this leaves her system. But if you see either sign, then we'd need to consider having her to her local vet for IV fluids to flush the drug from her system and supportive care to counter the effects of this overdose.
Please take care,