Walther LG300 XT Junior Air Rifle is imo the best option but you may be a bit put off by the price. Then there is the Hammerli AR20, but it really isn't a patch on the Walther. Then there is the Air Arms MPR, but it really is a compromise and probably a little too large for your purposes.
The Walther may be the most expensive but I feel that it would be best suited to your needs and probably best value in the long run.
Rutty