As mustang put it, follow your gut. I am in "her" shoes and am learning about this whole blended family thing as well. There certainly can be drama but it also can be a very beautiful thing. I'm not sure of her particular circumstances but there are several things that stand out that I wanted to share with you:
1. Take it slow, no matter how you feel in the moment it takes a long time to completely be over your divorce (no matter the situation). You have a lot of work to do and will need a lot of alone time. It takes discipline! It isn't necessarily getting over a former love but getting through a life change. Everything you do and have is different now and it takes time to adjust.
2. Don't meet the children for awhile - at least not until you know it is the relationship is the real deal. Kids need time to get through their feelings about the divorce (yes, at three - that is how old my son was) and if pushed into a relationship too soon they will resent them - no matter how much they also love them. Give your little guy lots of space. I dated my current bf right after my divorce for almost 3 months before he ever saw my son. He said sometimes he wondered if I really had one! We took it slow and when they first met he and I did not show any affection towards each other and called him my friend. We gradually became closer in front of him and it worked beautifully. When we would do things with each other I would ask my son if we could invite my bf. Now, they have an amazing relationship because he gives my son and I space and encourages my sons relationship with his father.
3. We learned that it was best for me to stay the disciplinary and for my bf to not try to be his dad. It is working really well. They have a very special relationship without the competition for attention or resentment. You will have to find your own unique relationship with her kids and vice versa.
4. Don't be afraid of your love (or like ;-) for her. Just be very disciplined in how you move forward in the relationship (sometimes you have to slow down when you want to speed up for circumstance sake). Accept her circumstances and allow her to be open about everything. It is difficult, trust me. But, I have been given that gift and it is the most amazing feeling in the world. I don't have to hide anything or be someone I'm not. I remain very close with my former spouse and I can feel good about it because it is accepted by my bf.
Sometimes your heart whispers....those whispers are best listened too.