The issue of being alone with an adopted child or kissing him or her is a
source of dispute. While it is true that many major authorites such as the
Chazon Ish and the Steipler emphatically prohibited yichud (seclusion) with
an adopted child there are other opinions and it is critical to consult
with your local Orthodox Rabbi.
Reb Moshe, (Igros Moshe E.H. IV #64.1 page 124.) for example, permits a
family that converts to Judaism to continue their relationships as before.
The father can be with his daughter and mother can be with son. Reb Moshe
says because there is no physical attraction between them. (Even though
according to halacha they are not actually related to each other.)
It would follow from this that in any case where there is no natural
physical attraction there should be no problem of Yichud. Some authorities
hold that this is true for an adopted child who was raised from the time he
or she was in diapers. It is important to note that this "diaper criterion"
is not discussed at all in the Igros Moshe - and this analysis should not
be used to conclude what Reb Moshe held concerning the "diaper criterion".
A general discussion of the issue of yichud and adoption is found in The
Journal of Halacha and Contemporary Society Spring 1987 "Contemporary
Issues of the Laws of Yichud" by Rabbi Azarya Berzon p107-112.
Dr. Schwartzbaum also descibes this issue in his book the Bamboo Cradle (pp
211-215). Rav Shlomo Zalman Auerbach had prohibited it.