Parental alienation takes a similar progression in most cases. The results can vary, and not all alienating behaviours of a parent lead to alienation of the child. Some children successfully resist the manipulation. Nor are all alienating parents intentionally behaving badly, they don't know what they are doing or the terrible outcomes that may result.
However, reality and perception are not necessarily linked. We buy things from sellers because of a perceived benefit that may not exist. Marketers do this for a living and we sometimes unwittingly buy into the marketing plan.
In short I can tell you that your kids most assuredly still love you and want a good and close relationship with you. Stay available; keep sending gifts and letters at appropriate times.
Don't hover or be seen to be overbearing, and under no circumstance discuss your understanding of what is going on with the kids.
2 places to get solid help are from
Www.warshak.com where there is good advise and an excellent DVD available,
Www.paawareness.org where you'll find a brochure of quick and useful tips to both read and give to others of influence over the kids (not to the kids themselves)