"Nature's Good Guys" - https://www.amazon.com/NaturesGoodGuys-Beneficial-Nematodes-Triple-Blend/dp/B086683PNS/ref=sr_1_3?dchild=1&keywords=Beneficial+nematodes&qid=1619006903&sr=8-3
There are several different types of Nematodes for different types of pests, these ones treats grubs and more. The "Hb" type treats for Grubs, and they do sell a package with only that type of Nematode. They sell them by the million and I believe the rate is 50 mil /12,500 square feet.
I'm fairly certain they're Chicken-safe, but I'm not 100% sure. I'm interested to know now, though, as we just got chicks. They won't be outside for another 6 weeks and I didn't treat any areas that are near their run, so low-risk even if they aren't chook-safe.