You would want to react based on your initial intentions of
knowing her.
IF you don't really like her but just want to be friends only
(LIAR), then it wouldn't matters if she's a mother of 1000 kids or
1 kid.
IF your initial intentions were to love and be together with
her, then you wouldn't mind if she got 1000 kids or 1 kid, cos
afterall those kids were produced by the woman that you loved and
want to be together with.
BUT if your initial intentions were just out for a fling, then
of course you would be upset, disappointed that someone else or
many other else (1000 guys) got to her first before
you.
It depends what do you really want out of this
relationship/friendship/any-ship with her.
But it would be WISE to understand that YOU WEREN'T IN
HER PAST TO SAVE HER, so why would you think you have any RIGHTS to
be upset about?
Since you think you should be upset with her past, then
Should she be upset with you for not showing up at her
past?
She had to LIE to you to protect both the happiness of you and
her, and when she had to put this happiness that were shared
by both of you at RISKS, do you still have any reasons to
doubt that she's not in love with you? 
well, if i'm in your shoe, after finding out that she is
divorced with a kid and if i love her, then i
will let her know that ok with me. i will also look after the kid
as if it is mine. then again i will also let her know that
i want a honest and sincere relationship with her. so i dont want
her to hide anything from me again.
