I hate to say it but I think sometimes the nap makes it worse! I have found that if you hit that moody part of the day, if you can get thru it, they get a second wind and are good to go and sometimes in a much better mood. I have on past trips gotten Sam to take a nap and found that it wasn't worth it!! lol!
Sammy 5 years and Peyton 10 months at Air and Space Museum, Washington DC