Hi pattir1, it depends on whether your children drink a lot of soda. If they drink more than 3 a day, then it would probably be a good deal. Remember, they cannot share the soda card (sticker) it is one per child. Also, if you'll copy the bb code into your signature, your ticker will work.
We were just on the Mariner in April and our 8 year old son chose not to get a soda card because he would not drink enough to justify paying for it. In the dining room and the Windjammer you can get orange and apple juice and iced tea for no charge. That is his preference over soda anyway. Milk is available in the Windjammer in the morning for no extra charge as well.
My wife always brings one of those giant insulated mugs with a lid and fills up with her drink of preference (usually lemonade) at the buffet and keeps it with her wherever we go. I won't lug one around and keep up with it but it does save money over buying bottled water and soda and such.