Then you wouldn't have liked some of the flight attendants we have had leaving BWI!!! LOL!!
Because basically they were looking for volunteers (offering free drinks) to move to put parents & kids together and these were older kids! 10year olds!! Had it happen on 2 flights this year! Flight attendant basically said we weren't pushing back from the gate until everyone was seated and we needed two seats together for this parent/child duo!! Finally someone moved in each case, but flight attendant was budging and we weren't talking 'little" kids either!
On various ocassions I have put my bag on the seat and the attendant didn't make me move it. I guess it is a case of YMMV!

My youngest is almost adult aged. I don't have to worry about travelling w/ little ones. But when a much older kid kept kicking the back of my seat recently (after the 8th time!) I Very Politely & quietly asked the parent next to her (in case she was special needs or something) if she could please have her stop. And the parent got VERY rude with me. When I got up to use the restroom I said something to the FA, he told me if it started again to let him know and He would put and end to it!! He was not amused.
Overall, I have no problem with the way SW works. On the flights I have been on, they do not make an issue about seating. AGAIN, YMMV. But if something is sitting in the seat most people don't have enough nerve to ask you to move it esp if it is a middle seat and next to a young child.