I host and am a traveller with airbnb and have used the site from when my son was 8 months. He is now 2 1/2 and I always put in 2 guests for booking and then message the host to ask about my son and what we would need to do. When he was small he didnt have a bed and used a cot which we brought and every host replied in a lovely way. Sometimes they didnt charge or they did half price as we brought our own bedding etc
As a host, I would like a function where I could charge for extra adults but not children as our house is already set up for families and the one thing I loved about my travels with airbnb were family kind homes where they knew we were having a holiday or stay on a tight budget and wanted to support this. Thats what I want to do for other families.
I think its important the host and traveller communicates. It would be great for hosts to adapt prices for their own expectations even if they are different from mine. I look at the number of beds and now he can use a bed I would search for 3 guests.
ps. I would always tidy the home at the end of our stay and when its our whole family even the 13 and 16 year old helps to make sure it remains how it looked before we enter it!