According to the ECtHR's press release:

The case concerned the conditions in which two children, apprehended when they unlawfully entered French territory in Mayotte, were placed in administrative detention together with adults, arbitrarily associated with one of them for administrative purposes, and expeditiously returned to the Comoros without a careful and individual examination of their situation.

The Court was persuaded that the administrative association of the two children with an unrelated adult had not sought to preserve the children’s best interests but rather to ensure their speedy removal to the Comoros. Placing them in a detention centre could only have caused them stress and anxiety, with particularly traumatic repercussions for their mental state. The French authorities had not provided for the effective protection of the children and had not taken account of the situation that they risked facing on returning to their country of origin.

The Court further observed that no remedy had been available to the children for the purpose of having the lawfulness of their detention reviewed. It reiterated that the fact of placing certain family members in a detention centre, while others were free, could be regarded as an interference with the effective exercise of the right to family life, regardless of the duration of the measure. The circumstances of the case, taken as a whole, led the Court to find that the removal from Mayotte of the children, who were very young (five and three at the time) and were not known to or assisted by any adult, had been decided and implemented without the safeguard of a reasonable and objective examination of their situation and had breached Article 4 of Protocol No. 4.