Just travelled from UK - California with a 23 month old and a 3 year old, which was, um, interesting...
Are you going alone?
The 5 month old won't be too difficult, as they mainly sleep. Feed (breast or bottle) on ascent and descent as swallowing helps them not build up pressure in ears.
Arrange a bassinet for 5 month old if possible. Not sure about seat configuration, but if there are 3 seats, and presumably your 2 year old has own seat, and someone else is sitting next to you, put 2 year old next to them, so you will have a bit of space for you and the baby.
Ask the airline for help with the baggage at baggage claim. They should arrange this, but if not, then put baby in sling and promise 2 year old chocolate or suitable bribe.
Going to toilet. Depends. If baby is asleep, then ask cabin crew to either keep an eye on baby or take your 2 year old to the toilet. This depends on the airline and how child friendly they are. I tended to take them both to the toilet with me when awake, but flight attendants offered, so I was pleased.
If you have a stop over, find a departure lounge where a flight has just departed and let your 2 year old run around.
Don't pack any sealed juices as they will be taken off you at security, but bottles are fine. I never had to taste anything, but be prepared to do so.
It isn't that bad. Really.
Good luck