I'm assuming from your username that you're in New York? If so -
https://www.borderlinepersonalitydisorder.org/new-york-state-partners-with-nea-bpd/ confirms that NY does host the "Family Connections" program which is for non-BPD caretakers of BPD (I don't think they discriminate on any other mental health woes, at least half of us in there are crazy and depressed, even from caretaking BPD if nothing else).
It's a 12-session program where a group of 5-15 parents/siblings/spouses of BPD work with a leader through a DBT program largely based on Marsha Linehan (videos available on Youtube). I found the curriculum portion to be a mixture of mindfulness and basic communication tactics like validation and boundary-setting - and not the most helpful part.
What's most helpful in my opinion is that the program spends half of each session just letting you all vent to each other and seek advice...and I cannot overstate what a relief it is. If you think this forum is good, wait until you drink stale coffee with a dozen people who don't bat an eyelash to hear the paramedics make snarky comments when you call, or the police were around last week, or that her therapist, you, your lawyer, her lawyer, the judge and two cousins are all accused of carrying out some daycare satanist sexual abuse of her so THAT is taking some time out of your year right now. I'd guess about a quarter of the people I met through the program also had PTSD going back to witnessing extreme violence from their BPD loved one - either towards the caretaker, towards themselves, or towards children - you can imagine the person relating "I can't open my mother's front door if she hasn't answered the phone without flashbacks to the time I found her covered in blood", etc.
Now, good news is that it's free, bad news is that during Covid it's just virtual.
I tend to agree with the "inability" to diagnose BPD in early childhood, simply because things like superficiality of affect or an unstable self-schema are actually relatively "normal" for children - and only indicative of a personality disorder if they aren't shed in later teenaged years.
As it sounds like you've experienced, what gets labelled "OCD+Histrionic+ADD" can just as easily be labeled "Bipolar+GeneralAnxietyDisorder" or "BPD+Neuroticism+Immature PD", etc. One you start layering mental health woes with overlapping symptoms, it becomes very difficult to start pulling apart the layers - even if professionals claim to be certain...their peers likely would've been equally certain of a different combination. All you can do it try to treat the underlying symptoms and keep an eye out for anything concrete like schizophrenia which will require a very specialized drug regimen unlike those for the other disorders for improvement.
As a total layman who is not a doctor...I'd say it's relatively good news that your child's "ADHD, ODD, OCD, Anxiety Disorder and Tourettes" paints a picture that is largely incongruous of ending up as BPD. There are two different forces pulling there (the hypomanic and the neuroses), with a lot of overlap - but hopefully none of them end up with severe mental dependency and abandonment issues, self-harm or violent outbursts in later years.
For what it's worth, I'm not convinced PDs, specifically BPD, are genetic; obviously you can find stories on either side of the fence or more usually just saying both predispositions and environmental causes are at play - so we each end up imparting our own experiences it seems. But in fairness my view has been challenged in the past year in discovering an adopted-out relative who never met my dBPD also has...we'll say some variation of the common-mother's mental health woes.