KIa Ora,
First things first, make sure 'you' tell them before a stranger does !
It's important for you to be the one to tell your children, especially if your appearance is beginning to change quite rapidly, where others outside the family start to notice and comment on it... You need to tell your children[depending upon their age] in a way/on a level they can understand...It's far better for them to hear the 'truth' from somebody they 'love' & 'trust' than to hear 'gossip' from a stranger...
Be honest and open and allow time for them to think about it and if they feel the need to ask questions-encourage questions keep the lines of communication open...Reassure them that your love for them will not change, only your appearance will change...
Perhaps you could let them know that you are/will be seeing a special doctor who will give you some medicine to make you 'feel' better...
If they are old enough to read, then write them a letter,explaining what's happening in simple layman's terms...This should open up questions....
Each of our journeys are different, and what might work for one person may be a disaster for another...
But remember: You know your children better than anybody else does, you have seen how they react to things...
Metta Zenda