Hi shcampbell,
I work in a pediatric office giving immunizations multiple times a day and do them much the same as moonchild described. We give Varicella and MMR SQ, and we also give IPV (polio) SQ as well.
Like moonchild, for smaller children and infants I use the thigh(s), stabilizing knees with one hand, and, when necessary straddling their crossed legs between my knees. (Those 4 y.o's can really fight!

) Mom or dad usually hold hands/arms/upper body for me.
For the older kids, say around age 11-12, I use the deltoid IM or upper arm SQ most of the time depending on the child and how much they're fighting. Amazingly, I had to hold down a 17 y.o. like one of my little ones she was struggling so hard!
Best of luck to you!
Donna