Start off asking everyone if they did something special today. I'm sure you'll get a whole range of answers, but if no one says partake in the sacrament, tell them this is one of the most important things they do all week. Ask them why they think it is so important? You guessed it, this is because it renews our baptismal covenants! We've actually been told that Sacrament is the most important meeting we have all Sunday.
3 Nephi 18:1-11 is, in my opinion, a really good reference to read for this. I wouldn't have them read this all aloud in Jr. Primary, but it's not too bad for senior. It's just a little long for Jr. I would think. If you want to use this for Jr. I would pick out maybe 3 versus to read and just fill in the blanks before and after. I know I've said this before, but for anyone new, my primary is pretty small so we combine Jr. and Sr. for third hour and do singing time and sharing time together. I usually struggle to find a way to keep both groups engaged.
Now that we've gone over how important the sacrament is, I thought it would be good to focus on what our thoughts and actions should be like during the sacrament. I've prepared a bit of an activity to get them thinking. I made word strips you can print and cut out (see my last paragraph about how to get these, I'm having technical difficulties at the moment). Draw a happy face on the chalk board and a sad face. Have a child come up and choose a word strip, read it, and figure out if it's a good thing to do during the Sacrament or a not very good thing. Tape it under either the smilie face or frown. I have 13 word strips so if you want to involve more kids, just think up a couple more things.
I also made a printable bookmark you can print out, laminate, and pass out for them to use in their scriptures. They can look at this during the sacrament to help remind them what their thoughts should be like.
I tried to upload a direct link to print these out, but it's not working. If you would like me to email the attachments to you directly, just comment leaving your email address and I will send them to you ASAP. Usually within an hour or two. I do have pictures above so you can see what they look like. Hopefully I can figure out how to get the direct links up again soon.
**Update 9/18/13 - I think I got the link up and working for these. This should take you to the Christ Bookmarks**