going from one toddler to one toddler and a newborn, we got a king sized bed from a queen sized. (we actually got a near new one from freesource, but craigslist is another good source for those not too squeamish about a used bed.) then they grew into a preschooler and a toddler so we decided to transition them into their own room with their own bed (to share if they wanted, or split if they wanted, 2 twin beds). one of the twin sized winded up next to the king sized in our room. now expecting another and seriously considering investing in another twin sized, just in case.
to be honest, we have a massive big bedroom, and its become literally the "bed" room. and the other 2 bedrooms in our house are just places to play. i second the reconsideration of buying the king. we were in dire straits when ds2 was born and couldn't afford the new bed, so when we saw the one on freesource we pounced on it. it has been such a HUGE asset to our sleep quality and overall happiness... to be able to share a bed without feeling cramped up. to be able to move around a bit. adding the twin next to us has been even better! (on a side note, we very rarely get gapping, but we have everything underneath the mattresses firmly together and we put sheets and blankets over to make it more like all one bed. if that makes sense.)