It's not impossible to catch it outdoors. Very low risk. However if you're kid is playing at the playground without a mask and someone sneezes in his or her face they have a decent chance of catching something.
As far as those being vaccinated not wearing masks I don't think we have the research yet to know if those vaccinated are still getting the virus and can spread it. Some of what I have read seem promising that they are not. However in the interim why not just wear it around others even when outdoors. No one knows you were vaccinated. Why cause them unnecessary stress?
I carry my mask outdoors and wear it when people go by. It may be an overreaction but so be it. I will also probably keep wearing a mask in stores and such even after the virus is largely gone. Seems a great way to avoid the cold and flu.