Difficult parents

On my paeds placement, I have been finding that it is the same parents that regularly fail to bring their children in for appointments. This one particular family have missed their appt 4 times in 3 weeks and when they bring their child in for treatment they constantly question what we are doing with them and why. Whilst it is good that they are asking questions, as soon as me or the senior PT explain why we are doing a certain treatment or that their child is slightly delayed in a certain area, the parents become very defensive. I completely understand how hard it must be to sit there and listen to someone say that your child is not doing things that he/she should be doing at a certain age, but it's not like we don't say it tactfully and we always point out the things they are doing well before hand. Due to the fact that they seem to miss more appointments then they attend, they don't do their child's HEP with them and everyhting we do with their child they don't like (no matter how much we explain why we're doint it), i feel as though my time would be better spent with a family that appreciate what we are doing for their child. Also, because this family dont attend regularly we are blocking up times that other clients could be getting treatment. In saying this, the child is the main focus, and they're not the one causing the issues. Just wondering if anyone has any ideas how I could manage the situation more appropriately (I have tried asking the parents what they would like me to work on with their child but they don;t know themselves).