Having the child lean his head back so the tongue slides back to the oral cavity. Sometimes having the child actually lay on his back is needed.
For kids who are fronting (remember what fronting is? If the child is fronting the K/G sounds he or she will produce them as T or D, i.e. a "front of the mouth sound") I have found that using a Dum Dum or Mini Tootsie Pop to told the tongue tip down and THEN tell the child to make the sound, will allow for the back of the tongue to move up and make the /k/ or /g/ or a close approximation. You could also use a tongue depressor but the sucker is more fun for the child!
Use gestural cues to cue children on which sounds we are working on. For the /k/ and /g/ I point to the throat. I will also lean my head back for an additional visual cue to get the tongue back and up.
Preceding a K or G initial word with the 'NG' sound is a strategy I use often. For some reason some kids can say 'NG' but not K or G; ex. for the word car model; NG...Car, then ask child to repeat.
A cough sound is made very close in the mouth to where the K/G sounds are. Replace initial K sounds with a cough to approximate the K. ex. model: [cough sound]......amp for 'camp', then have student repeat.
A rocket sound is very close to the K as well. Use the same method as above but use a rocket sound instead of a cough.