What is the most likely diagnosis for a child with a midline cyst in the neck that moves with swallowing?

The presence of a midline cyst in the neck that moves with swallowing is most indicative of a thyroglossal duct cyst. This type of cyst forms from remnants of the thyroglossal duct, which is a structure that normally disappears during development of the thyroid gland. These cysts are usually located along the midline of the neck and often become more prominent with swallowing or tongue movement due to their attachment to the hyoid bone.

The characteristic movement of the cyst with swallowing is a key clinical finding that helps distinguish it from other types of neck cysts. For instance, a dermoid cyst typically does not move with swallowing, as it is not connected to structures that facilitate such motion. Similarly, while a branchial cleft cyst may occur in the neck, they are usually lateral rather than midline and do not demonstrate movement during swallowing. Teratomas, although they can present as neck masses, usually have a more complex presentation and are not defined by movement with swallowing.

Thus, the specific attributes of a thyroglossal duct cyst—its midline location and movement during swallowing—make it the most likely diagnosis in this scenario.