Hi goat1, I'm in exactly the same position as you, but a year older. DS and DD same age as yours. I had a MMC last May, but have had the yearning for no.3 for a long time - we always planned to have 3, but after 2 sleepless wonders our relationship took a back seat. (Still didn't put me off mind you, I must be crazy
)
The MMC pregnancy wasn't planned, financially things were tight, so i'd taken morning after pill after a slip up, which didn't work. When I got my BFP, we were both shocked, but over the moon and I think after it, we feel there's a definite space in our lives for no.3, in addition to DS and DD constantly asking for a baby in the house!
We haven't been trying since the MMC, but have decided we're just going to see what happens over the next 3 months, not worry about date checking or purposely choosing to try. Of course, we were intimate 10 days ago and I'm now in 2WW and obsessively symptom spotting, unbeknown to DH 
I guess timing wise, DD will be at school full-time in sept, so I'm probably starting to feel a bit redundant - I took a career break (which am unlikely to return to) 2 years ago, and now am happy working in a bar 3 nights a week, so I feel I've made the decision already that I'm a home bird now, and I don't want this phase to be over just yet.
Hope you make the right decision for yourselves, good luck!