When we moved into our (at the time) badly-in-need-of-a-reno house, we had the ducting done. I watched them clean the ducting and saw the dust and nastiness being blown outside of the vacuum on their truck. It was gross.
Our conditions of our home were similar to you - years since and maintenance done, various pets in the house (we also sprayed the house when we moved in), and also a serious mold problem too. Now that we are finished we need the ducting cleaned again due to the construction dust and any airborne contaminants. If you child has breathing allergies you might want to keep the ducting extra clean to keep him (her?) comfy