I don't think the released version of Blockland was intended for children, they can play it, but there are things that came with this game that I don't think would have been included a game designed soley for children.
One of the old default saves that has now been removed was of a satanic church (DM Church). creep Bear and rooster Mongler faces are included as selectable faces anyone can use. The spray can also has swearing on it, although I believe it is censored. There might be more that I have forgotten about.
Kids can survive playing a game with the included points above, but it makes me suspect they weren't the only people this game was designed for, if at all.