A device can be sync'ed with only one iTunes account at a time. If you sync the device with your own iTunes account, and your son or daughter tries to sync the device with his/her account later, he/she will be prompted to restore the device to factory settings before loading music, apps and settings from his iTunes account to the device.
If your son or daughter has a separate iTunes account that is sync'ed with the device, and it doesn't have a Credit Card associated with it, you could do one of two things to purchase Mobicip.
1. If you think you can handle it, the best option is to use the Home Sharing feature. Home Sharing allows you to purchase music and apps once and use it across multiple computers and iTunes libraries. The setup for this is a little bit involved. Please follow the instructions in this tutorial from Apple
2. The other option is to setup a gift card or a monthly allowance for your son/daughter's account. For more information, visit this Apple page
If your child doesn't have an iTunes account already, you can create a new iTunes account and use a redeem code or a gift card in lieu of a credit card. Launch iTunes and sign in using your daughter's iTunes account. If you purchased a gift card, click the link that says "Redeem" at the top right of iTunes under Quick Links. Or simply click this link