Can my children be added to the trust later or what is the age limit to allow being included in it?
list them as beneficiaries or amend the trust later. It's easy.
Trusts are a no brainer right now. They are by far the best way to go with NFA. I put everyone I care about on my trust to avoid accidental felonies (e.g. constructive x-fer). Unfortunately Obama is pushing stricter rules. There are even a couple of "pro-gun" groups that are bartering away NFA rights to get what they want. So if you are going to act. Act now before the new rules go through. This will hopefully grandfather you in on all your current possessions.