We had a foster daughter who was born positive for methadone and cocaine. The problem was that the hospital she was born in had NO experience with methadone babies... so they monitored the cocaine withdrawal and discharged her at 5 days old. What they didn't know is that the higher the methadone dose, the later the withdrawal can kick in--up to 3 weeks post-birth.
In our case, she went through withdrawal in our home--unmedicated.
The first 6months are difficult. There are withdrawal backlash episodes of crying inconsolably (like colic) for a few months intermittently. You DEFINITELY have to limit their exposure to stimulus--no "coo, coo, coo"ing in their face, limiting light, noise, etc. for several weeks or so. This, of course, delays their ability to take in stimulus for development. As a result, they're GOING to be behind--but should catch up quick.
I only had the one and we had her from 5 days old to 10mo. She's now 3yo (as of yesterday). She was also suspect for FAS because the mother was also a drinker. In our research, we found that many drug-using mothers who weren't necessarily alcoholics would try to soften the blow of coming down from their high with alcohol. But this child's mother DID happen to have an alcohol problem in general. And obviously the methadone helped her keep off of heroin, but not cocaine.
Our ffd is a bright kid. She has some language delays, but nothing severe. She gets speech therapy and some behavioral therapy (but I honest to God wonder if this is a matter of her current family's method of dealing with her).
Not sure if this helps...