So, you let your child's sippy cup get moldy. There's no shame in that. They're such a pain to clean, with their little mouthpieces and removable parts. Or, maybe it's not your fault; maybe you just rescued that long-lost blue cup from the rubble of toys beneath your kid's bed. The important thing is you noticed the cup grew fungus and intend to clean it, and aren't still filling it up with apple juice. Fortunately, some soaking can do most of the work for you, so you shouldn't need to employ too much elbow grease.
Plug your kitchen sink drain or get a clean dish pan, and fill it with hot water and a small squeeze of liquid dish detergent. Make the water deep enough to fully submerge your child's sippy cup.
Take off the top of the sippy cup. Pull out the rubber stopper inside the lid, if there is one. Soak all pieces in the hot soapy water for at least 15 minutes. Drain the sink or empty the dish pan afterwards.
Wet a sponge with hot water and put a small dab of liquid dish detergent on it.
Scrub the inside and outside of the sippy cup and its top with the soapy sponge. Use a bottle brush, toothbrush or cotton-tipped swab to scrub in the edge of the bottom of the cup, inside the mouthpiece and in the groove of the rubber stopper.
Fill the sink or dish pan again with enough warm water to submerge the sippy cup and add in half a cap of bleach if you want to take an extra step to ensure the cup and parts have been completely sterilized. Soak the cup, lid and rubber stopper for 15 minutes. Put on non-porous rubber gloves to remove the pieces and rinse them off thoroughly under running water.
Place the lid, rubber piece and cup upside-down on a dish drain to dry.
Wear non-porous dish gloves while scrubbing the sippy cup if you have sensitive or dry skin.
Sippy cups are at increased risk of mold growth when they contain moisture and can't dry efficiently because they're mostly closed off to fresh air circulation. Even when you aren't washing your child's sippy cup, remove the lid and rubber stopper between uses to prevent mold growth inside.
Also, to prevent mold growth, make sure the lid and rubber stopper are completely dry before putting them back together.
Don't use an abrasive sponge surface to clean a plastic sippy cup, since you'll probably scratch it. Small scratches on dishes become havens for bacteria and mold.