Hello tigwig, I think as long as you are happy with every other aspect of her general health and well being then there is nothing to worry about. I have two daughters 6 and 4. The 6 yr old is fine about going to bed usually wakes in the night for a wee and gets up at 7, the 4 yr old however fights it all the way, she thinks she is missing out on something by going to bed. Sometimes she is up and me and hubby are yawning and desperate to get some zzzzz's so we end up putting her in with us, then to top it off she is up at about 6.30 !! Our problem is like you the little one still likes a nap in the afternoon and I feel the same as you why let her get all cranky and upset if she want's to sleep let her.
She may just have a lot going on in that little head of hers and needs sleep to gather herself for the next busy day. Sometimes dehydration makes little ones sleepy which is not so good, (not pointing finger at you) but just check she's got enough fluids. Just sounds like she's very content with going to bed and being in her own company too, lucky you.
x