Benadryl can reduce itching and hives but it will not treat the life-threatening parts of an anaphylactic reaction (suffocation from tongue/throat swelling and/or dangerously low blood pressure). Epinephrine is the only thing that can save you from anaphylaxis, and it should be given early to stop the reaction before it spirals out of control. Don't give Benadryl and think that you're safe, call 911 immediately on any sign of swelling or breathing difficulty or lethargy from low blood pressure.
All that said, you absolutely should give peanut butter (and other allergens) early and often. If you don't, you're signing your kid up for much worse odds of anaphylaxis later. The good news is fatal anaphylaxis is extremely rare even in people with confirmed food allergies, and even lower in infants than older kids and adults. Even when anaphylaxis occurs epinephrine is extremely effective, so anaphylaxis deaths are almost always due to failure to treat with epinephrine. So just make sure that you have access to prompt medical treatment if you need it (i.e. not camping in the mountains far from any hospital), and give those allergens ASAP.
All that said, you absolutely should give peanut butter (and other allergens) early and often. If you don't, you're signing your kid up for much worse odds of anaphylaxis later. The good news is fatal anaphylaxis is extremely rare even in people with confirmed food allergies, and even lower in infants than older kids and adults. Even when anaphylaxis occurs epinephrine is extremely effective, so anaphylaxis deaths are almost always due to failure to treat with epinephrine. So just make sure that you have access to prompt medical treatment if you need it (i.e. not camping in the mountains far from any hospital), and give those allergens ASAP.