Kittens, unlike puppies, can be potty trained very early. That being said, when kittens are 4 weeks of age and younger, they need help expelling waste. They need to be stimulated to release urine and stool; when kittens are with a Momma Cat, she does this by licking the area. Without a Momma, kittens need you to help them pass waste.
Kittens 4-6 weeks of age should begin urinating and defecating on their own. If you notice urine and feces consistently in your foster area, it is a good indication that your kittens do not need to be manually expressed.