The children who come to my Sunday School are typical upper-middle-class London kids - they have lots of school work, a full schedule of extra-curricular activities from ballet to Mandarin lessons, and busy social lives filled with birthday parties and holidays. It's important, therefore, that church provide them with time for peaceful contemplation and down time.
So here are a few prayer ideas that I've used that have worked (I won't include the time when I asked "okay, now what do you want to say 'thank you' to God for?" and one kid shouted "SLUGS!!!" and another shouted "POOING!!!").
1) PEBBLE PRAYER. For this, you need three baskets and a collection of pebbles or other small objects. Pass a basket full of pebbles around the group, and tell children they can take none, one, or two, to represent things that have happened this week. Have the other two baskets on your altar or focus space, and call the children in groups of two or three to bring their pebbles up, placing pebbles representing good things ("Thank You Prayers") in one basket, and pebbles representing bad or scary things ("Help Prayers") in the other basket. This can be a good gauge of the general mood of your group - if it would work for your group, you can follow it up with "would anyone like to share what their pebble represents?"
2) HOLDING CROSS. With the children in a circle, pass an object around - whoever is holding the object offers up a prayer (or says "pass," and hands the object to the next person). After each prayer, the leader can say, "Lord, hear our prayer" or a similar line, which all the children can repeat.
3) TAIZE CHANTS. I've started using Taize chants in prayer time. I put extra candles, and a few icons, on our altar - when it's time for prayer, several of the children light the candles, and another turns out the lights. I then ask children to share prayer concerns - these have ranged from prayers for sick guinea pigs to prayers for those affected by IS terrorism and Ebola. Then, together, we sing the Taize chant "O Lord, Hear My Prayer" three or four times.
4) WALKING. If you have a large enough space, get the children walking around it in a relaxed way, and then introduce a line to repeat - I've used "Lord, let my prayer rise up to meet you, as the day rises to meet the sun." Repeating this over and over, with the physical activity of walking, can help turn the prayer into an almost unconscious refrain, matched to the rhythmn of your walking.
Related product: Our palm branch cross has a lovely feel and texture that would work well for circle prayers, and is fairly traded.