What state you live in?
It may be Rose Slug damage... Check for little green worms on the underside of eaten leaves...

Since its still a smaller plant you can just put gloves on and take them off the plant...
Slid your hands on the underside of leaves...

For larger and more rose bushes I use Safer Insecticidal Soap on the top and underside of leaves... This soap spray must contact the Rose Slug to be effective... I found spraying in the evening after sun goes down works best... Follow directions...
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What you have is damage from sawfly larvae. Small green caterpillar eating the leaves (usually from the underside) Very common for most of us this time of year.They really don't do much damage other than some unsightly leaves and the population should be dwindling by now (at least mine are) If you are concerned, you can always turn the leaves over, locate them and squish them. Chemical control is often overkill as it kills the good bugs, bees also.

As you have already been told, what you have is damage from rose slugs or sawfly larva (two names for the same insect).

They've been very bad this year and for some reason, the Knock-out family of roses seem to be attracting them more than most. It looks bad, but the damage is usually mostly cosmetic.

The Knock Out family are resistant to black spot, which is a disease. They aren't resistant to insects - they are still susceptible to all the insect pests that attack other roses.

I'm a pretty lazy gardener, truth be told. Half my roses have damage that looks like that, covering a third to a half of the bush/plant. Nobody angsts over it but me... Hubs could care less. Visitors look at the flowers and the redwoods and say "ooooh, pretty!" and never notice the foliage damage. As many times as I have looked (a lot), I have yet to find more than 2 culprits, so I gave up. As long as my bushes are still growing, I can live with some unsightly leaves. I don't like it, but I can tolerate it.

Has anyone noticed the Knock Outs being resistant to aphid damage?? I had aphids on other roses in my garden but none on my Double Knock Out, Pink Double Knock Out and Sunny Knock Out.
In that photo above-- it looks like rose slug damage.

Aphids & rose slugs love our Double Ko's here...