You should NEVER inflate any lifejackets within the cabin.
Upon reaching the door an infant lifejacket can be inflated or assist a child in inflating theirs.
You must remember Ethiopian Airlines Flight 961 where due to passengers inflating their life jackets they died as they were unable to swim under the water and through the door.
I do understand it may be an airline policy for some airlines but would have thought they would have updated their policies back in 1996 when this happened.
I know my airline policy is no jackets inflated until exiting the aircraft and infant life jackets are not to be attached to the adult (using strap attached to the infant life jacket) until both are outside the aircraft.