I spent several days in Rovinj at the end of August 2015 but have only been through Opatija on the bus. My impression was that Opatija was a quieter place, a sort of old-line resort. You might Google for pictures. I could be way off base. I'm not sure how convenient "on the outskirts of Opatija" would be unless you're planning mainly to take day trips away from the city by car.
Rovinj has a medieval historic district that is made of stone. It's fun to explore but indubitably touristy. I liked the town and wouldn't mind revisiting it, but my preference is for places that are not quite that intensely given over to tourism. If you give me the address of the apartment you've found in Rovinj, it's possible I'll have an opinion about the location The bus station (in case you're not renting a car) is not too far from the medieval area.
Rovinj is well-located for trips to other coastal towns in Istria. Porec is equally touristy but has a different look--pretty pastel buildings. The little hilltop village of Vrsar is worth an hour or two. I think I was about the only tourist there the day I visited; probably everyone else was at the beach. Pula, which I didn't have time to see, has Roman ruins; it may be equally easy to reach from Opatija.
In 2015 there was a once-a-week (I think on Wed.) Inland Istria bus tour that could be picked up from Rovinj, Porec or Pula. It was really the only way to see more than one of the inland historic towns without a car. We went to Motovun and Groznjan and made other stops as well, including a winery. Inland Istria is very pretty, and it would be a shame to spend all of ones time along the coast.
If you want to check on bus service from Opatija, try the BusCroatia website.
Be sure your accommodations are air conditioned; you will almost certainly need that. With a four-year-old you are probably going to be looking for beach time, but unfortunately I cannot help with info on beach conditions. I think some of the beaches are rocky.