Welcome to the forum.
Other than the box bundle with pass there aren't currently any separate gift vouchers for the kids pass. However all the passes are contract free and can be cancelled at any time.
What you can do is start the free trial and then cancel it straight away by going to your My Account > My Passes page http://account.nowtv.com/passes and clicking the cancel pass button next to it. You will still be able to watch until the 14 days is up after you've cancelled and it won't renew.
Then when you want to subscribe again just come back and buy a kids pass at £2.99. Again you can cancel it straight away and you will still be able to watch until your month is up but it won't renew.
Just keep doing that as and when you want to use it.
i'd also recommend enabling your payment PIN to prevent accidental purchases
And if your children are particularly young and you have the entertainment or cinema pass as well, you might want to enable parental controls to prevent them watching unsuitable content
hi i added a two month kids pass, tried to cancel so i won't be getting charged after the two months expire, but doesn't let me! do i have to wait until nearer the time to cancel this two month pass? thanks