as far as i know, you dont need to do anything as long as the precocial birds are healthy and the mother duck is around. there were cases of swans or ducks walking on highways or busy streets. in this case you immediately call the police (here in germany they are responsible for animal safety too) and maybe, if it's safe, make the drivers aware of the birds. i can understand that you were worried, but in this situation you probably only could have waited and watched out for the ducklings with the smartphone ready. often we dont have this time, then we have to deal with our worries. if you have a local environmental group, you can write them about your observation and send some pictures. they might use this information when dealing with the city council, e.g. in their effort to lower the speed limit in certain areas. i guess there are also children playing in this park. they might be in danger too.
in the second vid the interviewed pensioner accompanied the swans for two hours and led them into his garden, i think he lured them with bread or chinese chews and they followed instinctively. i often carry one of those yellow security bicycle jackets with me in case i have to enter the street like the policeman does in the first vid
