Yes, we went to Tanto but considering how much water that Stockholm is surrounded with, I can’t say it’s very accessible. That is also the view of some friends living there.
If it's a beach that's appropriate for smaller kids you're after with shallow water, yes you probably have to go to the more prepared beaches with sand that has shallow water a bit out.
Many places in central Stockholm have been built out from natural beaches by claiming water so the water will be deep at those points (probably held quays that was used for transports before trucks took over), but even in many places like that nobody would bat an eye if someone took a non-nudist swim.
Apart from the extremes most local teens and upwards will find cliffs by the water, as mentioned not appropriate for smaller kids and seniors as it's adjacent to deep water but perfect otherwise.
