Child’s swing going sideways
I built a large swing for my grand children what I learned having the A frame legs wider at the base about a 15 degree angle and then having the swing supports , chain, or rope a similar spread did the trick.

Originally I had the supports the same width as the seat and they would end up swinging at an angle, I went and measured several swing’s at our local park and found that they were wider at top, after moving the brackets out the swings now swing true.

I had tried all kinds of things changing from chains to ropes changing the length I had even tried taking the base out of level (this was a dumb thing and a lot of work) 

After everything the wider at the top did the trick it kind of makes sense but I thought the swing needed to go straight up after measuring 2 different swings at the park I tried it and what a difference , it doesn’t affect their ability to get going or no one ever noticed but it did stabilize the travel.