Here's what I expect will happen: The team will recommend a course of action, presenting no alternatives. Very likely, your employees will express some level of irritation at your insistence on being involved when you clearly don't know what they know about the subject. That's OK -- be patient. They're just feeling their oats, so to speak.
Your job is to ask what alternatives they've considered. If they tell you there aren't any alternatives, draw a line in the sand, because there are always alternatives.
Don't, however, allow the situation to turn into an argument. If they start to insist, present a scenario: "Imagine for a moment the company is running out of cash. Whether we like it or not, big budget cuts are coming. Our choice is to kill the project altogether or find a way to deal with this design issue in a way that costs only 20 percent of what you're proposing. Are you telling me you'd kill the project because there are no alternatives?"
Most likely they'll explain that there really are alternatives, just not very good ones.
Which puts you on solid ground. Insist on the alternatives, and a comparison matrix that demonstrates why they're inferior to the one they're proposing.
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What you're doing, of course, is forcing your employees away from the emotional, my-team/your-team decision-making you're concerned about and back to the use of logical frameworks.
One more thing: If at all possible, for the first decision that comes along, do everything you can to help your employees discover a middle ground that legitimizes both your involvement and their new expertise. Doing so will do a lot to help them remember what team they're on.