no sponsor letter is required and is for all residents in Qatar anyone can apply i just had mine done today
you just follow the instructions above. one update is that it is done only in gate 2 as far as i know and costs as mentioned above
You can get an ID card for your children with valid RP from any Immigration office and then swap it for the smart card or you could also just try and go to the smart ID place directly and ask if you can do it skipping the obtaining of a normal ID card will save you the trouble and money