I have used Rosetta Stone and have looked at Power Glide and Power Glide Jr. I also minored in Spanish in college (it took 8 years of Spanish before I felt comfortable having a conversation with a native speaker.)
I chose not to go with Power Glide products because I didn't like the mixing of English and Spanish. I think it encourages too much translation. Plus, I thought they looked hokey.
I really like Rosetta Stone. However, I don't think your children will be fluent speakers when they are done. The only way that will happen is if they spend lots of time with native speakers where English is not spoken at all. Nor will they have a good background in grammar. It uses the immersion method - there is no English at all in the program. The program uses an ingenious mixture of pictures and Spanish so that they get the meanings, similar to how children pick up language. Children don't sit down with a grammar book and learn how to conjugate verbs or ensure noun verb agreement before they learn to speak. They pick it up as they go along.
For a child as young as yours, you might look into La Clase Divertida. I haven't used it, but I have heard rave reviews about it. I think Rosetta Stone might be a little boring for a child that young.
mom to 13yos, 10yos, 6yod
homeschooling from the beginning.