The vast majority of children will like nothing better than an
array of junk food - mini sausages, burgers, chicken nuggets (and
lashings of tomato ketchup), white bread and crisps, followed by
cakes, biscuits, mini-chocolate bars, all washed down with vast
quantities of fizzy drinks.
While this is a menu that strikes a chill into many parents' hearts, there are several good reasons for providing this sort of food: the children love it (and it's a party that is primarily for the children); there are very few children who won't be able to find something to eat given this sort of choice (many children are unbearably faddish about their food); it's cheap and takes virtually no preparation.
Even if you are normally meticulous about the food you eat, you might consider making an exception for a child's party. It will certainly make your life easier.
If the junk menu sticks in your gorge, you are confronted with the very real challenge of creating a meal that all your guests will actually eat. The likelihood of succeeding is minimal, so you have to accept at the outset that there will be refusals, rejections and possibly tantrums. If you are prepared to countenance this sort of behaviour, then the choice is yours.
If you run a strictly organic/vegetarian/vegan/macrobiotic household and you are taking your child to a party, you really should grit your teeth and accept that your child may be offered food that you would normally find unacceptable.
It is the height of bad manners to berate the host with a list of banned foods. It is highly insulting, implying that they are the purveyors of poisons (even if you believe this to be true). They will feel castigated for their choice of food, and may even feel steamrollered into trying to find an alternative (which could be the last straw for the frazzled party-giver).
Allergies and food intolerances are, of course, a different matter. If your child suffers from any sort of allergy or adverse reaction to food, then you should ring the host beforehand to let them know that this is an issue.
If the allergy is life-threatening, then you might suggest that you will send the child to the party with his/her own sandwiches etc. You could even offer to stay at the party (or return in time for the birthday lunch/tea) to keep an eye on what your child is eating. All these suggestions are perfectly acceptable if a real health issue is at stake.