I've been with my boyfriend over two years. We both have 2 kids each, ranging from age 7 to 14.
For a while we discussed meeting each others kids. Hopefully it would make family time more fun, we could go out for the day etc and also see each other more as currently we are both very restricted by work and have maybe one night per week maximum we can see each other without kids.
So taking it very slowly, he met my kids at the weekend as a 'friend'. It was very relaxed, bit of time in the garden, some lunch and a few games, then he headed off. We all seemed to really enjoy it, lots of fun and laughter, couldnt have gone better tbh (I know not all plain sailing in future though!).
I'm not really sure what 'pace' would be sensible and fair on kids? Maybe meet every few weeks for a few months just for the afternoon? Then if all going well then think about overnights?