To have a guaranteed seats together, it's best to check in online and select specific seats. Seat reservations can also be made when purchasing the ticket. Occasionally, seat selection will require an additional fee. Always check the carrier's terms and conditions. When traveling with a small child, it is the airline's responsibility to ensure that an accompanying person will accompany them during the flight. In the case of separate seats, you should report it to the passenger service at Checkin or Gate, or inform the crew on board about this fact or try to change with other passengers on board. If you care about a specific place, it is worth paying extra ;)