Is a girl's consent to marry a man who is already married with children, knowing that this might upset his first wife, considered right or wrong?

The Sharia permits a man to marry a second wife if he is capable of being just, and your consent to the marriage is not considered an injustice to the first wife. So, if he is a man of religion and good character, accept him as a husband. There is no harm in you accompanying him when he travels after marriage, and he must be just between you both, even if you are in different countries.