There's really wide variation in the market. See here:
Kinder Konto Vergleich. We're at our local Sparkasse - mostly for convenience's sake -- and are happy with the Bambini Konto that our child has. But Sparkassen in different towns and regions offer widely different rates and promotions, so if you have the time and energy to compare it could make a big difference. Online banks will offer higher interest rates but with other potential drawbacks. Sparkassen and other banks (at least where I live) sometimes offer a signing bonus of up to 50 Euros for opening a new (adult or kid) account, which might sweeten the deal enough if the rate's not great, but the bonuses are offered only at certain times of year as a special promotion (so it may pay to ask at the bank a promotion's on the horizon, if you can).
GL!