What adverb can I use to describe a slap that intends to get someone's attention?

The adverb "enticingly" might correspond to what you are lookinng for.

From SOED

entice v.t. Persuade or attract by the offer of pleasure or advantage
enticingly adv. in an enticing manner

I agree with @YosefBaskin. A strong verb is better than an adverb or adjective. Swat, tap, slap, etc.

You can also phase it more suscinctly, e.g.

“Listen to me!” Samuel swatted Cal's cheek. “...

You're writing in the present tense (says vs said) which seems to be in vogue at the moment but is tricky to do because it can lead to writing that sounds like a laundry list of actions.

Another thing to consider is who your point of view character is. If it's Samuel or Cal, you have an opportunity to give internals as a way to convey what the intent of the slap was or how it was received.