Congratulations. You must be very excited.
I am happy for you, but also very sad at what your 20 month old will be going through. Even the 3 month old. I think of how my son was at both ages and know how traumatic the change would have been for him. I'm so glad you are aware of the grief these children will face. Obviously their life has not been a bed of roses, but they can't understand at this point that they are moving onto a better life.
I am not knowledgeable about the specifics in these situations. However, I am a mom and I know that holding and hugging are what is most needed--for both these children. I would recommend getting good baby carriers as both kids will need to be held almost continually. Check out
TheBabyWearer.com There are a lot of experienced moms there who may have ideas on how you carry these two kids at the same time.
I will be thinking of your little ones for quite some time to come. I send you the strength to help them (and you) adjust during this very difficult time.