I would be very interested to know which airline or even which country you are talking about (I would be interested to look see who governs the airlines and their laws in general).
I travel alot mostly between the US and UK but I have travelled a fair bit to other countries too and you can reserve seats as much as you like but if there reasons (and good reasons) the cabin crew CAN and WILL move you for example... if you reserve seats in an exit row and are unfit/unable to operate the doors, you reserve a seat that is disabled accessable and someone needs to use that seat, you have a seat beside a power point that is needed for medical equipment.
Last time I flew with my then 15 month old daughter (just her and I... we always buy a seat for her too) she was required to be fastened into a car seat to use the airline seat... also because the car seat can cause an obstruction it has to be placed in a window seat or in the center seat of the center row... because we had been put in different rows and both aisle seats they had some guy moved so that her car seat was beside the window and I was sat beside her... like I said if the cabin crew need too they will move you, regardless.
Ash