I would give your new birds a few days to settle into their new place before having your child start interacting with them to avoid any extra stress. Once they're settled in, you can have your child start to interact simply by staying near the cage and watching them, or offering a special treat such as a fruit or veggie they may enjoy. Once they are calm around your child, you can then get the budgies used to handling by offering them a treat prior to petting or holding them. Once they are used to being held, you can have your child pet them while you hold them.
Depending on your child's age, it is best that you are there to monitor and handle the birds to avoid anyone getting accidentally hurt or injured. If you notice your birds are becoming stressed out or flighty, it is best to take a break and let them relax and settle down, as well as move to the previous step they were comfortable with before trying again.