1. To prepare souvlaki, combine first 5 ingredients in a large zip-top plastic bag. Add chicken to bag; seal. Marinate in refrigerator for 30 minutes, turning bag once.
2. Prepare grill to medium-high heat.
3. While chicken marinates, combine cucumber and next 4 ingredients, stirring well.
4. To prepare skewers, remove chicken from bag; discard marinade. Thread chicken and zucchini alternately onto each of 4 (8-inch) skewers.
5. Place skewers on a grill rack coated with cooking spray; grill 10 minutes or until chicken is done, turning once.
6. Serve tzatziki sauce with chicken skewers.
for your toddler: Remove 1 to 2 chicken and zucchini pieces from the skewers. Cut the chicken and zucchini into bite-sized pieces. Serve the chicken and zucchini with a small spoonful of tzatziki sauce for dipping on a rimmed plate.