I did some searching around and it appears NFS:MW for Samsung Smart TVs is indeed free.
You might want to setup a PinCode for your children.
http://support-us.samsung.com/cyber/popup/iframe/pop_troubleshooting_fr.jsp?modelname=&homeid=57329&from_osc=&idx=467060&
If you haven't added any payment information (i.e. CC, Paypal) to your samsung account you shouldn't need to worry about extra purchases. If you did, I would assume it will prompt for your password or PIN code to complete the purchase.
http://www.samsung.com/us/appstore/appstoreFAQPopup.do?faq_id=FAQ00000005
I have a sammy smart TV but I never use it's smart features ☹️
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What I would do is find an app that costs money, click on it, see what the prompt screen looks like. Hopefully it asks for a password to your Samsung account.
I'm not sure what all kind of apps are available for TVs... but you will want to be sure that In-App-Purchases cannot be made by your kids. This is the problem with phone apps, most of them are *free* but your kids can easily rack up thousands of dollars in in-app purchases for in-game currency. That is why it's very important to find out what the authentication screen looks like for the Smart TV.
I google'd around and I couldn't find any clear cut details on how that works. I'd be shocked if it doesn't have some form of authentication (which you don't want your kids to know, even if you trust them).
Hope that helps.
TL/DR
That game is free. You will want to find out what the Purchase screen looks like though. Ideally have no payment information on your sammy account if you never want to make a purchase with it.
