Reporting this for you:
Benadryl (diphenhydramine) and all antihistamines that cause drowsiness in children carry a substantial risk of seizures and respiratory arrest. Children have less tolerance for these drugs than adults, and even in adults the therapeutic dose and the minimal dose for major side effects are close together.
Which is why many old physicians like myself opposed making this drug over the counter. Too easy to get into major problems with a slight increase in the recommended dose or an interaction with another OTC medication.
I love my pediatrician because she is "old school". I would never give my kids more than an amount she said to give, and I only flew with them once on a round trip. The benadryl I don't think really helped them sleep so I didn't use it on the way home, but the lollipops were definitely a hit almost the whole flight.