Hello N,
If we go away for the weekend, Friday afternoon until midday ish on Sunday the toliet lasts us all weekend with spare capacity. We are 2 adults and one 4 year old. We use our toliet for everything.
We also store the caravan at home, so I empty the toliet down the sewer in the drive. I have found that once the "contents have travelled" they are more mushed! Maybe that is to much detail.
I know that most people would empty and drain everything, and I expect someone will soon complain about the excess weight being towed around! I guess it depends on your towcar and where you are storing the caravan and also how often you use it as some people drain the hot water and flush tanks every trip, where as I do not. Having a small child also means you have a mobile toliet behind you, and with flush and a tank of hot water you can use the toliet and wash hands when in a layby. It is also worth noting that for number two's its best to have the flap open!
As for overfilling, most toliets have a gauge and you can also see the level. In you manual it should say what to do if it does overfill. If you are unlucky to have this happen (like it did to us) you will only let this happen once! Good luck.