There are lots of gratitude journaling apps- this one is free and for IOS.
Noticing the world around and within us is a great place to start when it comes to creating a gratitude practice. Make it an "eye spy" game, wherever you are, look around with your child and try to name one thing for which you are thankful .
Gratitude is about recognizing both the everyday and special occasion "good" in our lives. At mealtime, pick up/drop off or bedtime, provide some of your own examples and ask your kids for some of their own too.