My daughter has been travelling overseas (9 hours flight) since 10 months old. She has been overseas 4 times already and she is 2.4 mnths old. She is kind of an expert now on the plane and fastened her seatbelt by herself as soon as she's seated. I think an interstate flight isn't so bad as long as you are prepared. Ensure you got things to entertain the kid. I doubt the domestic flight offers childrens gifts and toys as international flights do offer. So you have to take whatever your DD loves, ie, book, teddy, comforter etc. Remember to take her favouratie food on board too. The food on interstate flight are not suitable for 21 months old baby. Offer lots of fluids is essential too. If this is to be your DD's first flight experience, I am sure your DD would be very excited and very likely to be seated on your lap..will, at least on take off, taxi and landing. One last thing, if you can , try to book a flight that's not so full, so you'll have an empty seat next to you so you could seat your DD beside you. And request the first row behind the wall (usuallly with an sreen on) can be a help as those seats are wider than normal seats.
I hope these do some help to you. Have a nice flight!
Lucy, mum to Stephanie-12/11/03