I'm so sorry I missed your response earlier! Here are three possibilities. The long ones could be shortened for the 8-year-olds, if necessary, by omitting the portions I have put brackets around.
Psalm 55:17 Evening and morning and at noon I will pray, [and cry aloud, And He shall hear my voice.]
Luke 11:9-10 [So I say to you,] ask, and it will be given to you; seek, and you will find; [knock, and it will be opened to you. For everyone who asks receives, and he who seeks finds, and to him who knocks it will be opened.]
Philippians 4:6-7 [Be anxious for nothing, but] in everything by prayer and supplication, with thanksgiving, let your requests be made known to God; [and the peace of God, which surpasses all understanding, will guard your hearts and minds through Christ Jesus.]