Is using WD 40 on stained clothing a good idea?

WD-40 is just highly refined petroleum oïl distillate. It is very durable, slow to evaporate and super oily. It can be used to "dissolve" some greasy stains like lipstick, bicycle grease etc BUT it will not wash out in water very easily so in effect, you have to remove most (or all of it) before you can wash the clothes. There are other solvents that are better but if you want to use WD-40, spray the stain, work the fabric with your fingertips, cover in baby powder, brush off the powder,and repeat. Then before washing put dish soap on the ring caused by the WD-40, rub it in, wait 15 minutes and wash in the warmest water possible for the fabric with a good dose of detergent.

Yes it will remove lipstick stains

As lipstick is oil based

But not all stains , each stain requires a different treatment

Blood needs cold water so it wouldn't work on this type of stain

The best stain removal is neat dish soap rubbed in prior to washing

Dying will not work as the wax from the crayon will act as a shield. So you would still have the crayon marks. If the clothes are colour fast you can try Didi 7 or a similar product. A baking soda and lemon juice paste might also work. But again the clothes have to be colour fast. As a last resort you could phone a dry cleaners and ask them if there is anything you can do at home to remove the crayon stains.

I hadn't heard this one... but it might be a good choice for lipstick.

For food products, or blood, I'd prefer to try a liquid detergent with enzymes: rub into the stain, and let it sit for an hour before normal washing.

I would think you'd be putting another grease stain into your clothing. Try it on a rag first.

Some dish soaps work well pretreating, and Lestoil liquid is great.

That is not for clothing.