though I know I can give them something to sleep if needed.
Please don't! Most of the times it is not needed thankfully, but if you do "win that jackpot" and have to do an evacuation, having an altered state of mind is adding to the problem. At such moments one doesn't want an adult on board with an altered state of mind, let alone a child who is already more dependant on others in evacuations etc. Again; odds are highly in favor of never running into an evac (let alone it being one that truely comes down to acute emergencies) but IMHO not a combination you want to have on your hands in the odd ball chance it were to happen.
If that is not enough; PLEASE do the kids the huge favor of NOT having them try something new while in flight. One never knows upfront how an individual will react to any and all medications. Can be as desired but can also be very much undesired. Anything from becoming hyper, having a sensitivity feeling (very highly) uncormfortable which can be both a side effect and caused because of how the drugs work on the body (feeling of loosing control can be very scary for some). Even when trying beforehand and having the desired results there is no guarantee for what it will be in flight. Up there our physiology changes a bit and can cause different results than on the ground. Same reason folks can get drunk a lot quicker while in flight, you need to drink more liquids in flight than on the ground etc.
Easy to say don't, but what then right? I'ld focus on giving the kids an idea what to expect. You having more then enough options to keep them busy for a lot more hours than the flight is scheduled to be (so in case of delays you don't find yourself running out of options half way through). It can be as simple as playing bingo, with the "numbers" or objects (depending on age) the kids have to "score" can be found in the flight while seated. For instance; how many seats in a row, can you see a lady in a red shirt etc. etc. Have pics and/or videos of disney and get into planning mode with them. What do they want to do and why? Who is their favorite caracter and why? Drawing, watching a favorite shows on their own dvd (with headset so not to bother others) etc. etc.
As far as tics; you know what can be of help and what not. For some a new computergame can help reduce. If so; go out hunting for a second hand game that is new for them. Not the best bit of parenting for daily life but these once-every-so-many-months/years things are quite something else then allowing it in daily life. For a girl in my hometown, she can reduce some of her tics if giving into one of her other tics; singing. She has been able to do that singing in a whispering voice, so it would not be audible unless you are literally next to her and listening for it. Perfect way for her to let go if need be without being very much in the picture. Those you and/or him can not reduce or steer; so be it. Inform the FA that works your area of the plane about it beforehand if it is likely he will have tics that others might otherwise complain about (for instance verbal ones that are audible) and if need be; those seating around you. Be nice and understandble for possible "bother" to them but not unsure or as if it is a bad thing. It isn't period.
If you fear a tic of wanting to kick the seatback in front of him; talk beforehand to the agent at check in and at the gate. If the flight is not full, see if they can arrange an empty seat in front of him or a bulkhead seat. This would not solve the soundproblem (would be another thing to work on) but would take away possible ouch to other guests. Full flight? After take off, move him about 90 degrees in the seat with legs your way. Not the most comfy for you but not having the seatback right there in front of his feet and ohhhhhhhhhhh so close for kicking might help things.
