A pediatrician recommended Benadryl (diphenhydramine) for my toddler's cold. Is allegra similar? 24lbs

Yes and no. Benadryl (diphenhydramine) is a sedating antihistamine. Allegra is a non-sedating antihistamine. How old is your child? Typically you should not use these meds on children less than 2 yrs old. There are other things you can try such as bed elevation and humidifier at night.
Antihistaminics. Well- Allegra and Benadryl (diphenhydramine) --- both are anti histaminics. Former is non sedating and latter is sedating. Usually Allegra is anti histaminics of choice for older kids. For toddlers loratidine or Zyrtec should be tried. 24 IBS -- seems like you have a kid anywhere from 12 mnths to 18 mnths age. Either way-- 1 ml of loratidine should. Do the trick. If symptoms do not improve within 3-5 days, consult.
Fexofenadine. Both of these medications are not recommended for children under 2. However, in my experience the safer drug is allegra (fexofenadine) which is a histamine receptor blocker. I have personally recommended many times.