When I met the present Mrs Hughes her girls were six and four.
I always thought I was useless with children, but the opposite appears to be true. We have been togerhter now for nearly twenty-eight years, and we have a third daughter who is nineteen. I utterly refuse to do all the 'step-dad / step-daughter' nonsense. We have three daughers, and I will fall out with anyone who tries to separate them in this way. As far as i am concerned, I am not my older girls' father, but I am their Dad, which is far more important to them and to me.
For the record - anyone getting involved in a partner with children needs to establish a vital ground rule from day one - back each other up, always.
Kids will play one parent off against the other - they all do it, so with partners it feels more sensitive, and children will pick up on this and exploit it. Don't worry, that's just children, and nothing to worry about! The trick is to always back up the decision the other one has made - even if you think it is wrong, discuss it afterwards, alone. If a child sees that he can get mum or dad to side with him against the partner, the wedge is in polace, and they will hammer it home relentlessly.
My partner would always side with me, and never allow the girls to undermine me by appealing to her natural maternal instinct. It is hard for the biological parent, but it must be done for the sake of unity and respect all round.
Good luck with your choice, which should be the right man - regardless of his children - or not.