Our rules for putting someone on the sex offenders list is very screwed up.
An 18.1 year old high school senior boy having sex with his 17 .9 year old high school girl friend on prom night can put him on a sex offender's list. A woman going topless on a deserted beach can be put on a sex offenders list. A 50 year old man having sex with a dozen 5 year old boys or girls can find himself on a sex offender's list.
I wouldn't want to live near to the last one, but the other two are a "so what."
Cheap fencing. 4 foot welded wire fencing is the cheapiest. You can buy 150 feet and have a 50' by 50' fenced in yard. Home depot is probably the cheapest place to buy.
ADDED: If you are looking for in-expensive fence, use chain link for the small area in your back yard, then as finances improve fence the actual fence line with 6' wooden fence. I put up a 6' wooden fence around my one acre back yard in about 3 months of every other Saturday, by myself.
Also if you go down the fence lines, your neighbors should help with either the cost or the labor or both. My neighbors did.
Good luck to you and yours.