Glad to get this advice. My parents sent me all my old LEGOs a while ago and I’m still in the process of disassembling all the sets, though at least organized into plates, bricks, etc.
Fortunately I know they’re not truly dirty, though they’re still pretty dusty so a good wash will help.
Dishwasher (on low temp) also works fairly well, just don't use any (or a very small amount) of detergent. I like to put about 1kg or so in the "delicates" laundry mesh bags when I have a lot of used legos to wash.
Also need to reiterate do not heat dry or use hot cycles if you go this route :)
They're made of ABS, which doesn't begin to soften until a bit over the boiling point of water (~104-105 C). It's a bit close to risk it, but even the hot cycle might be fine as long as it doesn't go much past boiling off the water.
Definitely avoid cleaning with acetone though; that'll turn it into goo.
I've run duplos through the dishwasher many times with no problems at all. On the regular cycle with dishes. Our dishwasher (Bosch) doesn't have a heating element for drying, it just makes the water really hot and waits for the water to evaporate from the hot dishes.
Fair enough, I've used quite cheap dishwashers in the past for this - where the heating element is used to heat the water up. Due to general cheapness, I'm sure there were hotspots.
That said, under hot loads with an old/cheap dishwasher I've only had issues with the large baseplates and very large/thin elements typically found in Technic sets - everything else has turned out fine.
Fortunately I know they’re not truly dirty, though they’re still pretty dusty so a good wash will help.