Middle-aged child seat
If you could find a seat that did not have footrests or was advertised as holding a 7 y.o., ~45 lbs (~20 kgs) child it would not meet the ASTM International standard. It's over all safety would be questionable.

According to consumerreports.org:  


  There are no federal standards for trailers or bike seats. The AAP recommends that any trailer or bike-mounted child seat meet the safety standards of ASTM International. Look for a sticker on the product or the packaging that indicates compliance. It's also a good idea to buy trailers and bike seats from reputable bicycle stores and manufacturers. Be careful about what you might find in toy stores, because those items might not meet the same safety standards.


If a child seat is manufactured according to the ASTM International standard:  


  The carrier shall be manufactured free of burrs, sharp edges, and sharp points; when present, these shall be properly covered with protective coatings to prevent injuries. In addition, the carrier shall be equipped with a rear reflector, footrests, backrest, armrests, protective devices to prevent contact of hands and feet with moving or movable components of the bicycle, and belt or capturing devices to keep the child from standing in or leaving the carrier. 
  
  This specification covers child carriers that mount to the rear of bicycles in order to transport children or accessory loads with a maximum weight of 18 kg (40 lb)


An option might be a tag along.
It also reduces weight on the rear wheel vs child seat and bags.

This rig would work with a rack and bags/panniers


EDIT:
Further comments indicate the need to carry two children.
A tandem tag along might be an option that reduces the inherent instability of a large weight high over the rear wheel and still accommodate bage.