I agree with John's post. Placing a speaker in an enclosed space -- especially if the speaker is ported -- is almost like modifying the speaker's own cabinet. However, it's not the end of the world and you can do things to minimize the negative effects. You don't want the cabinetry to act as an extension of the speaker baffle, so like JohnK said, try to place the M22s just forward of flush with the entertainment center's face. Also leave enough breathing room behind the speaker (~ 3") for the port.
You won't really know how much the speaker's sound will be affected until you try it. It may actually be fine. Even if the sound is noticeably affected, the trade-off you're making for the sake of security sounds like it'll be worth it. Those kids are going to be spoiled with good sound.
