Yes, these can be harmful if ingested. But with most slime-making kits, there are warnings about not ingesting any of the chemicals used. This is why it could be advisable for a parent to at least go over the instructions with a child or to even be present when a child is making the slime to make sure they understand.
Also, with these, inhalation could also be an issue especially with glue, so it is always a good idea to have a well-ventilated spot or even to do this outside, where inhaling fumes won’t be a problem.