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Although your nanny of 18 years may not be able to provide care, if she is available testify regarding the care she had previously provided and attest to your involvement with your son that would be powerful. It seems to me that you went an extremely long period of time with no issues and only now when he lost a great caregiver this concern come up from new inadequate replacements. I would also recommend obtaining your sons medical records from his doctors to show that his health needs are being met.
If you have friends or neighbors who visited with you routinely I'd also have them on standby to testify on your behalf regarding all the efforts and sacrifices you make for your sons care. These situations can be difficult as you are attempting to prove a negative, ie proving that something never happened. The best way to do this is too prove what did occur.
I would think that you're for caregivers testimony would be the most powerful piece of information. In addition, any other family members that have had that chance to observe you with your son would also be extremely helpful.
I hope this information and advice is helpful and I wish you the best of luck.