Buu was already strong. Frieza even mentions him in the movie. His pops warned him never to challenge Beerus or Majin Buu. It would make perfect sense for Buu to fight him. That would definitely illustrate the huge difference in powers if Frieza were to make light work of him. And correct me if I'm wrong but does Buu even need to train to keep his strength? He was asleep or locked away for god knows how long and was still merkin cats.
And perhaps Goten and Trunks may have gotten weaker due to lack of training but at the end of the day they're still Super Saiyans and are much stronger than Picollo, Yamcha, Tien, Master Roshi, and Krillin combined. They should have at least been able to fight the henchmen. Would have been light work for them.
Also, they should have let 18 fight. She's also stronger than those betas. Krillin should have been on babysitting duty.