The reason these types of medicines were taken off the market for children under 6 is because children were dying. Please ask a professional this question. I would NEVER trust someone that I don't know and trust with a medication question. You don't know these people and they could be wrong.
Your child's Pediatrician could give you better advice too, they often don't use Benadryl but Claritin instead. Benadryl is not for every day use but more for an intense allergic reaction. Please be aware these drugs have a VERY DRYING effect and can cause your child to act out because they are in pain because they are too dried out.
If you decide to go ahead then only use 1/2 the dose. It is much easier to give a little more than to take it away because your child passes out and/or is lethargic hours and hours after taking it.
I keep Children's Benadryl in my purse because all I prefer it to pills and have a multitude of allergies. I take 1 teaspoon instead of the 2-4 teaspoons it recommends. The quarter doses work fine even when I am swelling or having throat issues. My bottle says to not give it to children under 4 years of age, ever, and to only give it to children between 4 and 6 on the advice of a Physician.