There is a short and sweet dua for Rajab, which is recommended to be recited daily, especially after the prayers.
To try and involve the children, why not print it out and stick it on some card, decorate it together, and then place it where they can see it everyday?
Depending on the age of the children, you can help them understand the dua by rephrasing it. I have attempted it here for the little ones, please do use it if you find it useful:
In the Name of Allah, the most Kind, the most Merciful
Oh Allah, bless Muhammad and his family
Oh He, from whom I hope for everything good, and I am safe from His anger at everything bad
Oh He, who gives so much even though we do so little
Oh He, who gives to anyone who asks Him
Oh He, who gives to anyone who doesn’t even ask Him or know Him, out of His kindness and mercy
Please give me, as I am asking only from you, all the good things from this world and all the good things from the hereafter
Please keep away from me, as I am asking only from you, all the bad things from this world and all the bad things from the hereafter
Because what You give does not mean You have less
Please give me lots from your bounty, O The Generous
O The Inspiring and The Distinguished
O The Bountiful and The Magnaminous
O The Gracious and The Mighty
Save me from Jahannam
P.S. Following on from this idea, Busy Mummys had the brainwave of turning it into a poster! So if you don’t think you will get around to decorating it with your children but would still like to have it up for them to read every day, check out our FREE download on the Buzz Ideazz website.