Its been a couple of years but when I worked in the airline industry the terminology "seated with" in regards to ANO's and other CAA regs can mean in a different row.
In a typical A320 or 737 you have rows of 6 seats. ABC one side DEF other side of aisle.
Parents could be sat in seat 27B and Child in 26B ....that is classed as seated next to. Aisle seats are classed as next to. I know when im sat in my row I cant see much of the row infront and wouldn't be happy with this with nervous or very young kids, so would pay.
Most of the Airlines I worked with- will always try to get the best seating options available but rarely at the cost of other paying pax - by which i mean, people who have pre paid seats. In VERY rare situations to comply with regulations and when all other options exhausted some airlines will move people who have paid for seats to sit parent and child together but that together could be like ive said above- different row and only as a last resort and with minimal disruption because quite rightly people who prepaid shouldn't be messed around. In other words we might move a lone traveller if there is absolutely no other choice, because pissing off 1 person to seat a child is better than pissing off 2 who have pre paid. This is why it can be the row infront or behind.
When seating a full aircraft complying with all regulations to do with extender belts, infant oxygen masks, emergency exit rows, disability considerations/ambi lift pax and accessibilty. It can be a nightmare. Families with young children are considered obviously, they are on the list of regulations to be considered but not at the top of that list, they are equally considered with all the other requirements and needs. Before check in/bag drop opens at the airport in some airlines check in staff are frantically working hard behind the scenes to get the aircraft seating compliant. Things change for people everyday between when they book thier flight and fly. Broken limbs, pregnancy, disability problems etc. All can affect where people can and cannot be seated on an aircraft. Its rarely a 5 minute job. In peak summer on full holiday flights experienced staff would spend over an hour trying to seat a flight compliantly with as little disruption to pre booked seats as possible. Its honestly like a huge jigsaw puzzle. Families of 3 or more will rarely get lucky to be sat all together without booking. Because when we do look to be compliant we only need to sit 1 adult with a child and when its a full flight or almost full, there are very limited options. Families are almost always split up in these situations, not for shits and giggles but for practicality.
Every airline must comply to the regs but every airline has a different approach. Some will pull out all the stops to please everyone as much as they can. Others comply by doing the bare minimum.
Airline industry changed 30 odd years ago from 1 ticket doing it all. Its now a pay per what you want. In most airlines a ticket buys you a seat...any seat. If you want 2 3 4 5 all together then just pay. If you extra leg room just pay. If you want to take more than a small handbag then just pay.
Having little kids means you get some consideration by the airlines but no guarantee. Young kids are just another "thing" on the list that needs to compliant.
This is a very generic explanation and maybe out of date now but hope it helps explain a bit more about what is behind seating policies. Im only trying to be helpful and offer some insight. I dont make the rules so please dont bite if you dont agree.
Another consideration this year is with Jet Fuel prices peaking - airlines are trying to save/make money where they can etc. So when it comes to extra charges, even the more family friendly airlines, I would expect to see a shift. Not so relevant to this thread but make sure you properly understand what your infant allowance includes if travelling with babies because some of my former colleagues have said where there was wiggle room any excess baggage now gets charged.
Happy flying 🙂