All Swedish words are divided into two groups: en-words (or utrum) and ett-words (or neutrum). Unfortunately, you cannot know to which group a certain word belongs but there are some tips to have a greater chance of guessing right.
Most words are en-words Most words designating a person are en-words ¹ Have a look at the ending, many endings take the same article (e.g. –a² , –ing and –het are always en-words) ¹ One common exception is ett barn a child ² The only exceptions are ett öga, ett öra and ett hjärta.
A child as the phrase itself implies no gender on the person compared to en pojke or en flicka. Below is a nice blog post helping to clear it up a bit for me.
It’s borrowed from Low German. You can find its cognates here: https://en.wiktionary.org/wiki/Appendix:Proto-Germanic/gabal%C5%8D