Rule of thumb is toddlers under age 2 must sit on a parents lap in flight. Two and older then the child must be booked onto there own seat - paying a full adult price.
When taking a toddler under 2 years the parent will need to be strapped in for both the adult and child.
If you intend to take pictures rather than simply enjoying the ride; then bringing a child along on your lap may be rejected as you'll become a nuisance.
When taking a toddler who must sit on your lap, its probable the child goes FREE so you won't have to pay for an additional seat.