You can't use the back jump seats for a car seat, so your only option is the front seat. Make sure the airbag is off and install the car seat properly, and if possible move the seat as far back as possible. Also, she's a little thing and young, I would install the convertible rear facing. A rear facing car seat is outgrown when the child hits the weight limit or there is less than one inch of hard shell above the head. At 20mos I'm sure she's still under those requirements (unless she's more than 35lbs and more than 43" tall! LOL!) She's must safer rear facing, especially in this situation.
But yes, its legal, just not recommended unless there is no other seat option for her, and like I said those jump seats cannot accomidate a car seat, so she can be installed in the front seat. Please consider the rear facing (google EXTENDED REAR FACING for more info on that)