Hypotonia Issue

I am the mother of 17month old twins. They have got hypotonia and are unable to stand, even when their weight is fully supported. They are unable to get themselves into a sitting position. They have recently started to pull themselves up onto their knees from their tummy. I wondered if this is a normal stage? Once they are on their knees they do not walk on their knees, they sort of flop onto the floor - should I be concerned?

Thank you.