I have 2 ducks in with my chickens and ice becomes a problem in the winter when they "play". I used a similar set-up using an old grill grate. I thought it might be hard on feet but the ducks just reach in to dabble or get a drink and their dribbles go inside where ice can form and not cause any problems.
On a similar note; I was wondering if anyone has used a stock tank de-icer in a baby pool. I am thinking of trying it but would use someone elses wisdom instead of trial and error if possible. My chickens seem to prefer drinking from the duck pool and then the ducks could still bathe and relieve their feet once in awhile. Only my second winter with ducks and chickens mixed so any suggestions are welcome.