They do not have to be show quality in order to be shown - but if the kids want to do well with them, they should be of the best quality available. It costs no more to feed a quality bird than it does to feed a substandard bird. Are your children involved in 4 H? If so perhaps the poultry club leader can point you towards some show quality birds. Good luck at keeping/getting your children interested in showing.
They don't have to be show quality. They don't even need to be purebred to enter and take them to the show. However, they need to be good quality purebred show birds in order to win anything.
The judges here will write comments on the cage cards. like "Nice!" on a bird that gets a ribbon. Or "wrong color" "bad comb". or "Poor quality" on the cards of birds that are not good quality.
I suspect that a critical comment on the cage card would hurt the child's feelings. For that reason, I recommend that you decide on a breed and invest in a pair or a trio of quality birds. Show birds are not expensive compared to the cost of any other hobby and a trio can provide you with a huge flock in less than a year if you collect eggs and start incubating.
Some people here will enter a bird just to get passes to the local fair. They don't care about winning, they just want the passes.
But overall, competition in 4-H and at the county fair is tough. The kids own good birds and work hard to keep them in peak condition. You child will be happier if his bird at least looks like the other birds and isn't visibly a mutt or junk.