You are not this child's mother. Fair or not, I feel you are over invested in this family. I understand why you are and you've gone over and above to bring these children forward but you are just going to continue getting frustrated without their support.
If they had different parents, I think these kids would make bigger progress. Not bad parents but lazy and enabling and if they had different kids, it likely be no issue. The combination of well meaning but ineffective parenting plus children with higher needs is not good one for child to given all opportunities. Sad but true.
It is just a matter of time before these children go school and not in your care. At that time, despite all your effort, the work will unravel with these parents.
You have two option - either continue on, fully aware that you are just delaying inevitable. Or for your own sanity, recognize you can not save them all and let them go. I think based on details you give, if parent were approachable and willing to listen to you and work with you not against, you would have already built that joint effort relationship in the last 8 years. Since you not managed to build that, it unlikely to happen now.