How to create a child safe area for the kids to play in?

We have a decent little are for the kids to play, they have there toys, playhouse, &children's pool in this area... It's Along the house and the shed is on the back side... We cannot put fences up in the area we live, except a decorative fence, i have a 1 year old son, & a 2 1/2 year old daughter... It is really hard to teach them the perimeters to stay in as the back yard needs to be leveled cause it is alleys flooded In the low spots, it will be fixed for next year so they will be playing back there next year... The area they are playing in now is going to have a house addition put there this winter so I do not want to spend alot of money on containment for this year I will be doing it next year in the back! So any ideas what I can use or do to close off a 30 foot by 15foot L shape for the kids to play in? Any help is appreciated. Thanks for your time!