I'm going to chime in as well.
I have played against kids. I really don't have a problem with it. BUT, like all firearms sports (and airsoft is no different), each person must be completely responsible for their actions both socially and legally.
A person under the age of 18 is not legally responsible for their actions, therefore, even if you find a field in ON which allows a minor to play (super rare), it is likely that the you as the parent must participate as well and be in DIRECT supervision of your child. This means, you two would be a team, if your kid gets hit, you call hit, and you both go respawn.
The above is a VERY weak work around in the rare situations where a veteran airsofter with multiple years of hosting under their belt has been allowed by the community to have their kid attend games with them. So unless you are going to be active in the community, don't expect your kid to be able to play until they are at the right age.
To wrap it up: Shooting in your backyard in many areas in ON is illegal by By-Law, so check your laws in your area, also inform your neighbors. I like to encourage firearm activities to all ages, but some sports have an age limits to protect everyone who participates, and again, airsoft is no exception.
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.