The alternative to a untiy candle for outdoor events is having a sand ceremony. To do this, you would have a large glass center container that is empty and four smaller containers, each containing a different color of sand placed around the larger container. You can purchase the sand at craft stores.

After you and your fiance exchange vows you would begin pouring the sand, then the two boys would add their sand and you and your fiance would end with your sand. The grains mingle together to create a pattern. You can then display the container of sand in your home as a visual reminder of the coming together as a family.

There are readings to accompany the sand ceremony that your officiant can read while you perform the ceremony.

I think it is wonderful that you are incorporating the sons into the ceremony. This is what my daughter did with their two young daughters at an outdoor wedding and it was lovely. They had a "wedding sand ceremony" instead of the lighting of the candles. We found the vases at www.graciousbridal.com. Her sand was ivory colored and his chocolate, which matched their colors. She poured a little sand in the center vase, then he did, and then they helped each of their daughters (his age 2 & and hers age 5) pour out of their vases. It was the most touching part of the ceremony and both of the kids were just beaming. She also bought the girls little pearl bracelets with their monogram from the same website. He adopted my 5 year old granddaughter, so my daughter had the new monogram engraved on the bracelet charm. Sounds like you are so thoughtful - the boys are going to be lucky to have you!
Best Wishes

Here is a link that might be useful: Wedding Sand Ceremony Vases

I love the sand idea! I'm going to search for the sand ceremony readings. This is the place to get good ideas!!

Whatever you choose, perhaps consider doing this some other time, not at the wedding itself. Even children who adore a parent's new spouse and thrilled about the marriage are going to have some mixed feelings, or at least stress -- that is healthy and normal. It doesn't seem wise to put them in the position of making public declarations at the wedding. And the glamour of being attendants or something is likely to be even more fun for them, especially for small children, anyway.

I understand the impulse to commemorate that you are all uniting as a family, as opposed to them being helplessly taken along for the ride on something that only involves the two adults. I think that is a lovely thing. But in the end, what is really happening AT THE WEDDING is that the two adults are marrying. After all, what if they should someday part -- what happens to the step-parent's vows to the children, and vice-versa? I worry when I read about things like a new husband giving a tiny ring to his bride's daughter at the ceremony. Unfortunately, not every marriage lasts forever, and if that one doesn't, what message is that little girl going to take away about the meaning of vows and rings? But that isn't the point; even in blended families that are going to live happily ever after, I just think it is asking too much to have children publicly make or receive vows or declare that they are "giving the parent in marriage" or whatever, even though everyone's intentions are the best and even though it may not seem stressful -- indeed, may seem thrilling to the children -- far in advance when the plans are being made.

So I would think about having a sand ceremony, or whatever you do, at another time, perhaps with a smaller group. Personally, I would find that so much more intimate and special anyway -- letting the family ceremony be its own special moment, where the kidz, not the parents, are the stars, as opposed to a secondary thing to the main event of the wedding.