What can I do to help my bonus daughter?p

How can we get help for my bonus daughter? I have a 6 year old bonus daughter. I’ve been with her dad and in her life since she was 15 months. She is behind in her development. She is very “immature” for her age. She is also behind in her learning at school. Her teacher recommended she get held back (in kindergarten) because of how immature she is and how far behind she is with her learning. A little more background context… Last school year, she was at her mom’s Monday through Friday and we had her on the weekends because we lived over an hour away from her school and then we did 50/50 week to week in the summer. We just moved to the town her school is in so we will do 50/50 all year round. She gets babied at her moms house. Her mom doesn’t make her use silverware when she eats, let’s the grandparents there give her a sippy cup, gives into everything she wants anytime she cries, etc. Her dad and I have been working hard to help with her learning. We do sight word flash cards the school Gave us, reading board games, lots of reading books, help her sound out words to try and read books, etc. We have been encouraging her to be more independent (go grab a towel herself if she spills (no she’s not in trouble for spilling), grab her own snack, open her own water bottles, wash her hands herself, etc) to try and get her maturity level to where it should be.