What causes a knot in a child after tick bite?

Tick knots. Lumps felt after a tick bite could be several things. First is a granuloma, or organized bodily response by a large cells in the skin. Accumulations of cells can be quite firm and take a long time to go away. Sometimes we inject them with a long-term cortisone to remain in the spot and treated for 3 to 4 weeks in order to get it to soften..
Inflammation. Any inflammatory response can lead to the sensation of a "knot" under the skin. This common reaction is seen after vaccinations in muscles. Any puncture wound made by a needle, an insect or otherwise, can occasionally trigger an immune response. Part of this response is the release of compounds that widen blood vessels that leads to swelling. This is usually contained to a small area, i.e. A knot.