40 is the new 30, A.! You're not old. I say go for it. I wouldn't even question having another at 40. Several of my friends had kids at this age and slightly older.
I had my one and only baby--now at 16mo--at 44. I thought, at my age, that ship had sailed for me. We weren't even trying, so it was quite a surprise. (My husband, 53, has two kids, 28 and 24, from a previous marriage.) We're both pretty well preserved for our age, and in good health. Pregnancy and delivery were smooth.
I didn't have kids at a younger age, so I can't compare and contrast the differences between early and later childbearing/childrearing for you. But since you have two toddlers, you're familiar with fatigue. LOL. I do find myself getting very tired many days, and I've had to start taking much better care of myself than I think I would have had to in my 20s or early 30s. But that's a good thing.
Also, be prepared for mild-to-moderate disapproval (for lack of a better word) from friends and family. This, of course, depends on your extended family and social circle, but many on my husband's side made us feel a little bit like we had lost it because he was essentially starting over again--and I am certainly getting a late start at this. I sort of felt like a I imagine a pregnant teen might feel--in most people's eyes, someone in their 40s shouldn't be pregnant any more than a 16yo should be pregnant. But, again, since you have two very little ones, you may not experience this.
I do know a couple of older moms who have been mistaken for their child's grandmother. That hasn't happened to me (yet), but I keep myself ready for the day it does--and I keep my grays touched up regularly. LOL.
We had tried unsuccessfully to get pregnant when I was in my early 40s. If we had had any luck, I would have tried for two. I don't think 40 is at all old, but that depends a lot on how you've preserved yourself. But at 44, I know I'm done, and my husband definitely is.
Good luck with whatever you decide.