If you are not planning on boarding last and reducing the time they have to be in a convined space that way; totally in the back!
For the kicking problem you could have an adult in your party sit in front of them possibly. I'ld go for the back as the bulkhead can give a feeling of being even more cramped in. The wall right in front of you does that to many people. It depends on how your kids react but I've seen that being a problem for some with these needs. The back would come with one huge benefit; not having 100+ people walking past your seat/row to get to their seat. It feels very crowded, make quite an amount of noice (lots of talking, yelling, kids having meltdown, suitcases bumping, overhead compartments being closed and opened), people will knock into your seat with their body or suitcase, it's just basic chaos pushed into one small piece of floorspace. All of those factors alone can be enough to trigger a melt down etc.
Oh and a nice one; bulkhead seats in the isle come with the risk of being hit by someones carry on. "Prefferably" just an arm or something, but heads being hit is also a risk. It's amazing how many people have no proper control over there carry ons. If you do go that route; do not have the kids sit in the aisle seat during boarding or deplaning!!
In the back on the other hand, people will be coming closer towards you if the are sitting in that area, but you will not be swamped with people all around you standing, pushing, talking etc. Only those designated to be there, will tend to be there. In the front, you will see each and every one person on that flight esp. when preboarding.
I don't have ADHD or autism, but if I could have, I would've gone for the back without a doubt! I use a wheelchair and preboard using an aislechair. Preboarding is less hassle for me as boarding last and I can only fly when sitting in certain bulkheadseats. I need to be carefull as I can get a sensory-overload and want to melt down (can keep up the social desired behavior but do melt down on the inside). I can tell you; it never fails me for the boarding of a flight to cause me to overload. Deplaning is the same, if not worse as people are even louder by then. Combine that with any bumping of my seat or me to cause me serious pain and it's no picknick during boarding or deplaning.

. By now I can only fly business, which on some flights means you will only have to deal with business passengers boarding via the area my bulkheadseat is in. An amazing difference in sensory information and even that sometimes is challenging.