I work with the pediatric population. I approach vaccinations based first on age and then on their body language/response to getting a vaccination. For instance, all young children (especially 6 - 7 and under) I automatically have someone help hold starting at about 6 months. I choose the hold based on patient age and comfort. Most children around 9 when we start giving the HPV vaccine will sit nicely on their own and allow you to give it. If they seem nervous, I'll ask their parent or someone in the room to hold their other hand. Very seldom do we have to hold children for vaccination at that age. You do have people regardless of age who are not going to cooperate with vaccination due to anxiety and fear. That's not always specific to age, but more related to personality and phobias. I've had 4 years old sit and take a vaccine without help, holding, or flinching and had to hold 14 year olds down like they're toddlers. But in most cases, giving a vaccine to a 14 year old is very similar to giving it to a 34 year old. Very few people even as adults like getting a shot. It just depends to what degree they display that nervousness or displeasure to others so it's almost always a case-by-case basis.
As far as quantifying the pain level of shots, I can't say that I can tell much of a difference between most of them. The HPV vaccine I always save for last because almost everyone tells me that one hurts. The others are very dependent on the patient's pain tolerance in general.
As far as making it easier, I don't draw it out with explanations, but give them choices where ever possible. I basically go "You have to get a shot today. Show me your muscles. Look at those muscles. You have great muscles. You've got this! Which arm do you want it in?" If they are 4 - 5 years old, I'll give them the option between arms or legs. I've noticed they'll usually pick the thigh. I'll ask them questions about school or talk to them through the administration about anything but the administration and then tell them good job when done. Even if they were awful, I'll tell them good job! In very young children, we just get the process over with and they immediately get a sucker. It would amaze you how quickly most kids stop crying when they see it. In infants again we make the process as quick as possible and then they are cuddled and comforted.
After care is very basic. I inform parents that mild fussiness, mild fever, and not feeling so well is a normal part of the shot process and can last up to two days. Anything other than that, they need to let us know about. Older children, I tell they may feel a little sore, but then encourage them to stay hydrated, busy, and to try to go about normal activities. Distraction works wonders. If they've had multiple vaccines from a catch-up schedule, I'll usually prescribe some Tylenol or Motrin as needed but in general we don't use it for routine vaccinations unless the patient is very uncomfortable.