Those are seed dahlias and will not get as tall or as large as you expect. At least not this season.

If this were my planting bed, I would not have lined them out in a row as that seldom presents a pleasing aesthetic. At the very least, I would have opted for a staggered planting. But if you don't want to dig up and redo, all I can suggest is to fill in with a low growing, spreading annual.

Yes: baby dahlias like these are never going to grow tall or fill in between that wide spacing.

Hi Natasha,
You've got a beautiful bed to work with!
Welcome to gardening! Wave petunias would be beautiful planted with your dahlias. It is more pleasing to stagger annuals and not have soldier straight rows. Good thing is you can easily dig up and replant. The Drift series of roses are beautiful,low growing and so easy care. They'd be lovely along your rocks with complementing annuals.
If you're in a Southern region I highly reccomend Southern Living's garden book.
Best of luck and have fun with it!

Thank you all so much for the advice. I really appreciate it.

Unfortunately the nursery did not have many options, I ended up planting a few Dianthus around the dahlias... Hopefully I didn't mess the flower bed up even more!

I think it will be fine.

It will be fine. Give it time and add annuals for this year while you plan for next. I cannot tell you how many mistakes I have made in my gardens (and I continue to make some, too-ha) in 40 years. One thing to note about petunias, if you decide to add some, is that they require constant deadheading. They are gorgeous, but to keep them blooming, you absolutely must deadhead.

Supertunias don’t need deadheading.

https://www.google.com/search?q=supertunia+garden+answer&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari

Thank you all so much for the help! These are photos I took today, I am hoping they will get bigger soon :)

I was actually going to recommend dianthus. They are low-growing, moderately-spreading perennials that bloom multiple times through the season and provide lots of color in sunny areas. Flowers are small but plentiful.

If you want the flower to be covered with flowers you will need a fast growing annual. The dahlias and dianthus will not grow fast enough to fill up the space by the time frost is here. I saw on another thread that you added portulaca/purslane, which should help. The only way to get the bed covered with blooms is to pick up several six packs of blooming annuals from the friendly neighborhood big box store before they become desolate (it is surprising how the nurseries become desolate and empty come summer). Trailing petunias, Lobelia (erinus), Geraniums (pelargonium), begonias, periwinkle, impatience are good choices.

please someone help! something is wrong with my dahlias....I thought maybe I just needed to deadhead them but i am having to dead head every single day....and I realized this is happening to even the small buds! they are drying/shribling up way too early. something is not right. Please any advice someone could give to help me save my dahlias...this is happening to almost every single one of my dahlias. I do water them regularly....they at least 6 hours of full sun a day... the weather has been over 70 since I have planted them but below 90..I know this isnt lack of sun or water...something is wrong and its taking every single one of my flowers no matter how much I deadhead

The plants pictured look fine. The flowers are not supposed to last forever, they will shrivel up eventually and their durability is dependent on the temperature. In warmer temperatures the flowers will start deteriorating faster than in cooler temperatures.

I agree! The foliage is very healthy. Those blossoms are just spent.