Congrats! Cute little birb, quite surely a blue tit (Cyanistes caeruleus). If you can, try to disturb them as little as possible during the few week it takes for them to raise their offspring. As long as the parents are zipping in and out, they likely find enough to feed their young and don't need any additional support. Blue tits will only eat seeds and nuts as adults and mostly during the winter. The chicks need small invertebrate prey like insects and spiders. So other than offer some meal worms, there's not much you can do. Once you see the fledglings hopping around your balcony, some seeds may be fine but stick to smaller stuff like poppy and avoid the big and heavy stuff like sunflower or peanut until winter. Here's a bit more background if you speak German: https://bremen.nabu.de/tiere-und-pflanzen/voegel/voegelnhelfen/27691.html