I think you've adopted a good ploy, whizzing some veg into their tomato sauce, fresh fruit and vegetables have always been very important to me. My granddaughter is only 18 months and her favourite ingredient seems to be the carrot, broccoli also goes down well, so I think we're all happy about that. As regards fads, eating something on one occasion but not on another, I'm aware that she is still teething and I think that effects her enjoyment of food some days. As she gets older I expect her mother will cook with her and I think that involvement with a meal is an incentive to eat.
Sarah - what confuses me is that they love something one week and woof it down and then don't like the same thing two or three weeks later!!! Tomato sauce and pasta seems to be the only dead cert and I have to confess to whizzing in steamed carrots and celery to the tomato sauce to get their veg count up ssshhhhhh! (ages 1,3,3 and 5 and they eat with us every Saturday!!!)
I wonder what tips and comments other members have about feeding our grandchildren? I'm a cook and recipe writer and my children were always fed on home made food. My experience with my only grandchild so far, is that she prefers the food her own parents make for her, to mine. Some of the dishes I have made for her have gone down well, and others not so well. My son and daughter in law assure me that the reason is likely to be that their daughter has good eating days and not so good eating days, but I wonder what experiences other Grans have.