I agree about the kids' bikes. My daughter's 50cc GasGas has got to be one of the most annoying motos I have ever heard. Geared low, with a torque converter auto clutch, it has to rev to make any forward motion, but the silencer (like most kids' bikes) is so short as to have very little effect on actual noise output.
As for the full-sized bikes, I've heard Montesa 4Ts with aftermarket silencers that reach very close to the point of obnoxiousness. I don't find any of the stockers too loud, and as someone else noted, the Beta four-stroke is very quiet.
"Coming back to where you started is not the same as never leaving.” ― Terry Pratchett (RIP