That would depend on what the issues are with those exercises. if its something like struggling to keep your feet on the floor during sit ups then you can place your feet under a weighted object or in the gap between a sofa/chair and the floor to give you a sort of ground anchor. If the issue with the sit ups is a range of movement kind of thing like how far up you can come i would recommend bringing your knees up in to a 90 degree position and performing a 90/90 crunch (you can find demos online, i also do them in some videos). the 90/90 crunch allows you to focus on contracting the upper abdominal muscles with a small range of movement and does not require the feet to be kept on the floor which some find makes it easier to initiate the movement. With the flutter kicks if its a strength issue then you could perform a static leg hold instead where you simply raise your legs inches off the floor point your toes and hold for the duration, and then begin to add in the kicks as the strength develops.
As far as burning fat goes your main objective should be engaging as many muscle groups or large muscles as you can, as this will burn more calories and result in more fat being burned. obviously the ab work is important to develop going in to the future but as far as the fat burning effect goes the abdominal work wont be the highest priority.
Hope this helps and if the issues are something else then feel free to get in touch and i's be happy to help
Cal