they like damp conditions but usually come in from the garden.
Things to look for:
dampness (in a kitchen is usually caused by plumbing leaks, may be under the floor)
entrance way - probably an air brick, may be broken, or holes in wall where kitchen waste pipes have been put through and hole not made good
Kitchen unit kickboards can usually be pulled off without difficulty. They are usually clipped to unit legs if you have them, or screwed/nailed to edges of sides of units if not. Pull yours off and have a look.
If you have a wooden floor, suspect damp under it. An old house may have bare earth, not concrete. Either way it should be well ventilated to prevent it getting damp (but see above re plumbing leaks).
Once you have got the kick plates off, clean out whatever you find, and you can sprinkle slug pellets. Once you have put the kickplates back your child will not be able to get at them. Sprinkle them rather than putting in heaps. This is easier for slugs to eat, and more difficult for DC to grab a handful. You can also put pellets under the floor through the holes you find, and sprinkle outside the wall where they are getting in. It takes time for the local population of slugs to drop, but it will over a matter of weeks, then you will only get the adventurous few that are willing to travel.
The damp is a worse worry since it may be causing the joists or floor to rot.