Have you discovered you somehow left damp clothing mixed with something metal (pretty easy for kids to do!) and have rust stains on your clothes, there is some help to get the stain out. You can purchase rust stain remover products, but there are some methods you can try at home.
In a small, clean spray bottle mix equal parts of lemon juice and water. Gently shake and then spray the rust stain to completely saturate the area. Allow to sit on the stain for 30-40 minutes, then wash as usual. Check the stain after washing (but before drying!) to check and see if the stain is fully removed. Repeat as needed.
An alternative method is to make a thick aste with cream of tarter and water. Rub the paste into the stain and allow to set for half an hour. Then wash as usual (no need to rinse the paste off first). Again, check before drying, as heat will always set stains.
If these methods do not work, there are commercial rust remover products available. Just be sure to use a product that is designed for fabrics, as some rust remover products are designed to be used on metal, concrete and other non-fabric items. Rit Dye Rust Remover is one that is safe for fabric, and you can buy it for about $2 at JoAnns etc.