I think you're right to think annuals. Early August isn't the best time for many perennial blooms, and with only six months lead time, new perennial plantings won't look fully grown unless you buy really large specimens.

It is probably best for them to go to a local garden center for advice in the early summer, but we can make some suggestions. Is the garden sunny, shady, part-sun? How large of an area? What colors? Any chance you can give a photo?

My suggestions assuming they're gardening in Full sun, or with beds that receive an average of 8 hours of sunlight a day.

Depending on how they want to use the garden they should consider zinnias, daisies, poppies, sunflowers, hibiscus. You could try the edible annuals (any food item) but the best looking ones in the heat of early August are the nightshades (potatoes, tomatoes, peppers, husk cherries, tomatillos) and the cucurbits (cucumbers, summer/winter squashes). August is also when you can start sowing the veggie garden with annual seeds to have a late autumn/early winter garden show, of peas, beans, radish, kale, onions, carrots...

You should give perennials a try even though they won't be the stars of the show. They will formalize the garden plot, and next year your son and daughter-in-law won't start from scratch. Some easy perennials that stay relatively small are Nepeta, Pennisetum, any of the Mums, Anemone, Hosta, Day Lily, and on and on. All of these will look lovely in August! All of these will bloom in their first year, so long as they're cared for and properly sited in re: sun exposure.

One of the nice thing about annuals like cosmos, cleome, zinnias, sunflowers, scarlet runner beans and allysum is that they can be started from seed and so that will help the bottom line. They will want to do soil prep so that the plants grow their best.

If they want to splurge on a shrub, Hydrangea paniculata varieties bloom in August and there are usually ones readily available at bog box stores and nurseries. Limelight is probably the most common. They are rock hardy (z 3) and not fussy as long as they get at least a half day of sun. They will need regular water the first year.

I can often get easy-to-grow perennials in super-sized 6-packs at one of my local nurseries and they bloom for me the first year - Red Hot Poker/Kniphofia, Helenium/Helen's flower AKA sneezeweed are two that I've gotten there that bloom in summer.

Zinnias and sunflowers were my first two thoughts

I'd add perennial Dianthus/carnations to Persimmons' recommendations. I keep garden records on my laptop each growing season. Here's what I noted back in 2014: dianthus b. 'Sooty' bloomed 5/28 to 6/25. I garden to attract & sustain pollinators and to have something that blooms nearly non-stop year-round in New England and sustains bees, butterflies and hummingbirds gets my vote.

Liatris spicata blooms in August and I bought a bag of liatris bulbs from Lowes very cheap. They bloom in the first year but tend to be tall and like full sun. Nodding Onion is also a late summer bloomer.