Past relationship history

Hi I was wondering if someone might be able to give me some advice on what to do. Im a 28 yo guy and my partner is 25, we've been in a relationship since October 2012, she has 2 kids from a previous relationship but the father has passed away. Her 6 yo old son lives with her and her 9 yo daughter lives with her mum (who she doesn't get on with) We had a child together September 2013 (my first) and I have not just looked after my own little girl but look after her other 2 kids to, I support her with everything she needs and my family also helps out as I have a large and close family. Her having had babies so young, while still at school herself did ring alarm bells and originally I thought she was promiscuous and someone who just wanted to live off benefits as shes told me shes had a threesome with 2 men and is always forward in talking about sex. I know its wrong to judge a partner by their past but sometimes its a defense mechanism to protect your own heart. Me and my partner genuinely get along great, she says she would never cheat on me and she always wants me around, I love her, she has been honest with me, even giving me her facebook password which I never bothered even looking at until recently. I found old messages from ex partners and people that were stating what I originally thought in regards to her promiscuity. This worried me but the weird thing is, she hasn't given me any reason to believe she will be disloyal and I know she loves me, I wonder if I'm just being insecure. Cut a long story short I have helped her to get a job working with kids with behavioural problems as that's what she wanted to do, but I am now worried as she'll be working with a man, just the 2 of them alone in a house looking after these 3 kids. She has to do a sleep over every so often to. This is very unsettling for me, I'm worrying that something will happen with her and this guy and I've started to make a contingency plan, as silly as that sounds. Do I just be quiet and wait for something to happen then leave or am I fooling myself being with her and should I just leave now, or am I just being insecure. Any outside perspective and advice would be welcomed.