New to all this?
Start with Small Garden Games for more small-garden game ideas.
See more activitiesFootball tennis for the garden needs a rope, feet-only scoring, and one worked ruleset for two players.
A washing line strung between two fence posts works as the "net", the ball must cross it and land in using the foot only, and a fixed touch limit keeps the rally moving. A rope between two fence posts works exactly as well as an actual net, no special equipment needed. A simple handicap keeps it fair when a young beginner plays against an adult.

The core idea in one paragraph
Football tennis rules for kids come down to one core idea: using the foot only, the ball must go over the "net" and land inside the marked space, and a rally continues until someone fails to return it. It is genuinely simple once seen once, which is exactly why a no hands garden football game like this needs so little explanation before the first point starts.
A few minutes to set up, then play
A garden football tennis net game only needs a rope or a washing line strung across the middle of the space. The garden's own edges, a fence, hedge, path or patio, mark the sides and back of the landing zone on each side.
String a rope or line at roughly waist height. Between two fence posts, chairs, or anything stable. Height matters more than exactly what material the line is made from.
Two touches maximum before it crosses, one bounce allowed before each touch. One touch to control it, one to send it over. This keeps rallies short enough for a young beginner to follow without losing track of the point.
Serve underarm-style with the foot, from anywhere on your own side of the rope. A gentle first touch to start the point, not a full kick, and no fixed service spot needed.
A worked scoring system
First to eleven points, win by two clear points, is the simplest scoring target to run with. Every failed return, hitting the net, missing the landing zone, or using anything other than the foot, gives the point to the other side.
Give the younger or newer player an extra touch, three instead of two, or the benefit of the doubt on any ball that's close to the garden's own edge boundaries, so a genuine beginner still has a real chance in most points.

For a bigger group, or a narrower garden
A parent-and-child pair plays against another pair for a bigger family gathering, using the same touch limit and scoring rules, with either player on a side allowed to make the required touches.
A garden too narrow for a line across it still works, using a fence as a no-net wall version instead. The ball must clear a marked height on the fence and come back before two bounces, trading the net rule for a rebound rule without losing the core feet-only mechanic.
Quick answers
No, a rope or a washing line both work fine, as long as the height stays roughly consistent along its length.
They describe the same basic idea informally. This page's house rules, two touches, serve with the foot, are one workable version, not an official standard.
Give the younger player an extra touch, or the benefit of the doubt on any close call near the garden's edge boundaries. A small handicap keeps most points genuinely contestable.
Keep going
Three places to go next, depending on what the garden actually offers tonight.
Start with Small Garden Games for more small-garden game ideas.
See more activitiesKeep Ball in Garden works on keeping the ball inside a space rather than a rally over a line.
Try Keep Ball in GardenGarden Penalty Shootout Game swaps the rally for a straight shooting and goalkeeping contest.
Play the shootoutTonight's five minutes
The handicap matters more than the exact scoring rule.