I don't think powder is useful for that tbh and a lot of people are against powder in general. Moisturisers are much more effective against skin conditions like rash, eczema etc. Physiogel moisturiser is the brand doctors usually recommend so you can try that
I wouldn’t put anything actually.. don’t want to clog up the pores even more. I’d give my baby a bath or wipe down and keep baby cool. If it’s truly heat rash, it’ll clear up