We were exactly the same, two kids, boy and a girl 2 year age gap. This feeling of wanting another never left me, I live abroad so feelings of loneliness creep in and I thought constantly about expanding our family. Anyway, we left 10 years from the second and endless discussions of pros and cons we decided to try for a third. Got pregnant straight away. The age gap and being 10 years older might have something to do with it but it has been really, really hard. You do forget the exhaustion and it nearly killed me. I'm in the toddler stage now and it's bloody hard, constant battles, whining, no peace. I'm ashamed to say I do look back and think I should have left the status quo as it was. I feel like I have years of schooling, play groups etc to do all over again, all that stuff and I'm bored and regretful.
Sorry, I'm sounding very depressing. But it's really hard going back to the baby stage. The good thing though for you is that yours are still quite little and the age gap wouldn't be great. I think honestly, that until you do it, you never know what it will be like, that's the thing about life. If someone had shown me a magic mirror where I looked into the future of today, I probably would have said no bloody way!
The one thing about having older kids though is that this little one in many ways was like a first baby but with all the benefits of the wisdom of knowing the time does go quickly (I keep telling myself that anyway), and to enjoy the moments. I did love the baby stage in many ways, and felt more patient and relaxed but the tiredness is hard. You have kids in preschool/school so that's a bit easier.
So I don't know what I'm trying to advise. In fact I don't think anyone can advise. I'm just sharing my perspective. I'm finding it really hard at the moment, but maybe in another 5 years my perspective will be different. I hope so! Good luck.