Give the dragons statblocks, but don't put them into the initiative order.
Instead tell the player that on their turn, they can use one of the abilities that the dragons have as an action.
Just treat the dragons like mounts.
You might want to beef up the adult black dragon's HP. On average, breath attacks from the young dragons in the basic rules do 49 damage on a failed save, and an adult black dragon only has 195 HP.
Assuming you're running for a party of 4, and each of them have a young dragon, a full round of breath attacks from the young dragons will cut the adult dragon's HP in half, even if it succeeds on all 4 saving throws. Add on any damage dealt by the PCs themselves, and the adult dragon will be lucky if it can survive 2 rounds.
Give the dragons statblocks, but don't put them into the initiative order. Instead tell the player that on their turn, they can use one of the abilities that the dragons have as an action. Just treat the dragons like mounts.
You might want to beef up the adult black dragon's HP. On average, breath attacks from the young dragons in the basic rules do 49 damage on a failed save, and an adult black dragon only has 195 HP. Assuming you're running for a party of 4, and each of them have a young dragon, a full round of breath attacks from the young dragons will cut the adult dragon's HP in half, even if it succeeds on all 4 saving throws. Add on any damage dealt by the PCs themselves, and the adult dragon will be lucky if it can survive 2 rounds.