I did it last summer and my twin girls (9 at time) were really into Frozen then. Was it worth the price... honestly, no. I wouldn't do it again. We did get a good reserved spot for the parade, but it seemed there were plenty of good spots anyway outside of the reserved area. The dessert party was good, but looking back, it seems really expensive for a couple of cupcakes and fruit dipped in chocolate. There were no seats for the party and we stood for two hours. You had to line up early even to get a table. My one daughter was exhausted and ended up sitting on the ground and wanted to leave before the fireworks. Last summer they didn't have a FP+ for the Frozen Sing-a-Long, so being able to reserve a show time seemed like a great perk. We really did enjoy the Sing-A-Long. Now that there is FP+, there doesn't seem to be a point to that. With the Frozen package, it seemed like we were always waiting around, i.e., checking in early for the parade and waiting maybe about 45 minutes in the hot sun for it to begin, checking in early for the dessert party and waiting around a long time for the fireworks to begin. It seems like that time could have been better spent on other things. Also, the price has gone up since last summer.
However, if this is your one extra and it would make your girls beyond happy, then maybe. I've booked a couple of extras for my upcoming summer trip and the worth it factor is really a personal decision.