Rescued 2 One week old kittens. Peeing but not pooping.

Hi! I’m in desperate need of help. It’s my first time rescuing kittens and I really want them to survive. I hope this forum could help me.

The kittens still have their eyes closed and ears slightly folded. I’m guessing they’re only a week old or so.

The kittens have only been with me for around a day and a couple of hours. I have been feeding them every 2 hours with diluted infant milk since it’s the only option I have. There’s no available kitten formula in where I live.

They have been peeing a lot every after meal but I can’t make them poop. I’ve tried to stimulate them several times but no result. I don’t know what to do. I hope someone could help me. I’m really worried.