@ Mommaginger
My DD had a MRI with she was 2 1/2. It is really going to come down to whether or not she will be able to cooperate. She will need to be inside the MRI machine itself (she will probably be in as far as her belly or even knees depending on how tall/short she is). If she will not be scared, if she is able to be completely still- then they would not sedate her.
If she is sedated, they will do it via IV. They will start the IV when prepping her. For us, they actually gave DD the sleeping gas. We were able to be with her up until that point. Then after she was asleep, they had us leave, they started the IV and we waited outside in the waiting area. They also had a pediatric anesthesiologist to administer her drugs.
Ask ahead of time to clarify if hers will be a standard MRI or with contrast. The "w/wo" contrast takes twice as long. If she is doing it with contrast she WILL have in IV even if she stays awake.
The pros to being awake are that you avoid all the risks from anesthesia. The cons would be just whether the child that young can do it. I know some parents who did have 4 yr old do it awake.
In our case, with the anesthesia and contrast, it was about an hour and forty minutes that we were waiting out in the waiting room. I tell that to you just so you can expect it- they told us it shouldn't be more than an hour, so, i was terrified when we were going past 90 mins that something had gone wrong.
If they have the anesthesia there will be small recovery time. As in 20 mins, maybe more, but usually not unless something goes wrong. They will give you aftercare instructions. Expect the little one to be tired afterwards, and it is normal to have puking too, if sensitive to the anesthes.
HTH