In a lot of cases, like here, it's more important that the timbre is soft and somewhat childish (using toy instruments like the xylophone, toy piano, but also 8bit shapes (mostly pulse waves and squares) and bitcrush, or sounds prominently in "cute/adorable" games). But, for the melody, I think I got a good idea of it: Mostly trying to use playful rhythm that you repeat over bars for which different notes are used, so it feels familiar and maybe a little childish, but at the same time varied In a lot of nostaligic beats, it's important to use the sharp 4th and then the 4th, but it seems it wasn't used here. Often a minor key too but here seems not to be. If it's a whole EP or album, referencing previous songs is somewhat important to try to do too! Especially when using slower tempos and much softer synths or samples. As you may observe, a lot of times doing counter melodies that are subsets of the main melody but at higher intervals, often octaves, 5ths, and 6ths! A bit like harmonising with singing! Talking of singing, using vibrato and portamento/slide notes is important in making it playful! As you may hear, it also needs to sound light-hearted in a way, hence how none of the instruments are played low (except the bass), since most of it should happen within the high-mids, a bit like a toddler's voice. It has to be somewhat simple too and spaced, so not dense in notes but also with notes that have a short yet with noticeable release. I think that's mostly it. Was that helpful?
