Hi K.,
It sounds like a pretty ambitious party. I agree that coloring pages and crayons might not be as enticing as one would expect, especially as you have very young children coming and will need something for them.
One thing most mixed-age groups of kids love is making collages on sticky paper. At our preschool, we use clear con-tac paper (two matching pieces, the collage is sandwiched in) ; companies like Constructive Playthings (google them! it's worth it!) have packages of 'collage boards'--paper with a sticky adhesive on them. CP also sells collage kits, as likely does your local store for things like pompoms, googly eyes, sequins, even cut-up squares of colored paper...anything that might stick is fun. This is also more novel than coloring pages, which aren't much different from what they might already have at home. My toddler groups used to love this sort of activity, too.
Or you might just have some toys available for kids to play with and move right to snacking first thing, as kids sometimes get too busy to eat and then sugar-up on cake.
In regard to another/different item to put in the eggs, I have discovered that parents LOVE when I put stickers in them instead of candy. We did this years ago at a church egg hunt and parents thanked us; add to this that families have different feelings around sugar, and this makes it all the better.
When my husband thinks I'm overthinking things, I am. If it were me, I'd just keep it at streamers and the activities you are already planning on. Three activities may be overstructured as it is for a group of this age. Cake before presents is right; I know some families that actually "do" presents after the party, because youngsters can have a hard time with watching the birthday child open a pile of gifts. It might be worthwhile to have a little activity book to give--Dover Books makes some great, inexpensive books, which you can also order online.
I think there's a propensity to overdo the kid birthday thing altogether. We see these coordinated-down-to-the-napkins parties, where the parents are burnt out before the festivities have begun. If we go the whole nine yards each time, it ups the ante for the next year. Get what you can online, see if you can hire someone to come in and do a quick housecleaning beforehand..or even do it outside if you can, weather permitting, keep it simple and try to have fun.
And Happy Birth Day too you too, mama!