Benadryl, no. But melatonin, sometimes, yes. My rule is to have a hard cutoff time after which it's better to take melatonin than to continue the cycle of whining, sleep deprivation, and next-day misery. The cutoff is late enough to have plenty of time to try all the other things involving wind-down rituals. It is not an every day thing. I found that having the consistency actually helps establish the rituals too.

Also, bedtime trouble usually means not enough outside time and physical activity during the day. Or that the kids want more of the parent's time.

Look into the research behind melatonin use. I’m not a doctor and certainly long term use of diphenhydramine is associated with neurological problems in old age, but I’m not sure melatonin should be used as a simple hypnotic as you are suggesting. It’s natural but so is testosterone. Hormones may not be good to tinker with. I say that as a long time user of melatonin. At the very least you may want to stick with lower dosages- nothing over 1mg which is about as low as is easy to find.

Kids versions don't come in anything higher than 1mg and one can make it a half-dose quite easily. It's really more of a last resort thing, and definitely not for every night's bedtime. How last resort? Maybe once or twice a month. Now that my kids are a bit older and bedtime rituals are established it's even more rare.

I realize that some parents reach for it every night and this is not something I'm suggesting.