If I booked tickets with a child, are we guarenteed seats together or do I have to purchase seats?

Is that airline seats you are talking about? Generally they would give you seats together, but I once had separated seats with my child (very young at that time) on a domestic flight and only noticed on the flight. But people are kind and moving so we could sit together. The flight attendants sort it out in that case. They will not let a young child sit with strangers and the parent sits somewhere else.