If it were me, I'd want to send the supply of medication for the children just so I would know for certain that these meds were there with them. Although dad says he has the OTC meds, you can't really observe that, can you? You don't know if he has enough, whether they are the right strength, or even if they are expired. For your own peace of mind and to remain in compliance with the order, send the meds yourself, along with a clear, detailed list of how, when they should be administered.
For the clothes, are you certain what dad has will still fit your children? With how quickly children grow, your kids may have outgrown what dad has there, even though the clothing fit during their last visit with him. I'm sure the guidelines don't require you to send an entire, complete wardrobe, and if you don't get everything back, don't send their really good things or special outfits. Pack a week's worth of undergarments, one pair of pajamas, and for everything else, send things that fit now but you know aren't going to fit anyway come fall/return to school time. You can also hit up a resale shop to get shorts, tops, and a jacket or sweatshirt for each, along with maybe one dress outfit if they attend religious services or are going to an event which requires such clothes. That way, you're not really losing anything special if these items don't make it back.
You don't say how old your children are, but if they are 10 or above, they can certainly be responsible for packing up their clothes at the end of the visit with dad--even if they have to pull them out of the laundry.
Don't make this a power struggle with dad, and then he won't be able to make a big deal out of it. Just do what you need to do to ensure the kids have their OTC meds and a week's worth of summer clothes. Easy peasy.