Your anxiety is practically jumping off the page to me, and not in s healthy way.
I mean, she's 3 and you've never been more than an hour and a half away from home? She only of away from you a couple of hours a week? You think she is too young to travel!!
I don't know if this is post Covid anxiety, or you've always felt like this, but she's at a perfectly normal age to travel. Many babies including one of mine) travel as newborns. Lots of families take holidays abroad in the first few months. Its really not a problem.
Your child will be with their other parent, so no, I wouldn't have any hesitation about that. I'd miss them, but if I couldn't go as well, then I wouldn't try to stop them. I am concerned about why their dad has spent so little time solo parenting them though.
As for whether they should be going because of your high risk pregnancy/whether you could go as well, thst depends on the nature of the risk.
High risk could mean anything from (1) consultant led because of increased BMI, or because you had a difficult birth with your first neither of which would make taking a short flight and issue (2) on strict bed rest for the duration of the pregnancy because your cervix gas a tendency to spontaneously dilate.
And having several early first trimester losses, whilst incredibly sad, is different in terms of flying risk, to you going into second trimester labour several times.
Only you know what the risk is like, and whether this due to your general anxiety, or because travelling really would be a risk.