Shanaz, both Grace and Zara are a similar size to Seren, so try not to worry too much. Also, I have heard that extra fruit will go a long way to making up for the veggies thing, at least she likes one healthy food group right?
Don't forget, pasta sauce is tomatoes, and have you tried grating some carrot or something into the sauce? Can you get a bit more creative with that?
Also, if she loves spaghetti, that might help if you do stir fry with noodles in it, I do stirfries with rice vermicelli, Zara likes those. Would cheese sauce on the veggies help? And remember, kids eat when they need to eat, don't worry about skipped meals, just try to cover the food groups with snacks also! Finally, have you ruled out teething if she reluctant to eat lately?
I think if you don't push the issue, they come round eventually, Zara goes through phases of barely eating then eating what I consider to be loads (but really isn't compared to other kids LOL).
Re the apple carrot celery juice - yes I make my own, that is probably my fav combo. I was using 1 carrot, 1 apple, and some celery, but I needed extra for Zara, and I added only extra apple and carrot, so it is a little sweeter, she likes it better that way. LOVE my juicer!
