What harm can it do to teach creationism?
There is no harm in schools teaching both evolution and creationism and letting the children make their minds up.
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Children’s brains are extremely porous and they can easily cope with learning multiple ideas and theories simultaneously, even when the theories disagree. They are able to reason and make up their own minds.
Whilst a questioning approach is a desired outcome of education, instilling a disbelief in science is perniciously dangerous. Encouraging children to disbelieve scientific theory may appear innocuous for ideas such as creationism or a flat earth. But the increase in climate change deniers and anti-vaxxers threaten the welfare of society and the environment. Giving a story like creationism equal credibility with scientific theory in an educational establishment has significant repercussions. This is especially noteable in the USA. In contrast to being at the forefront of a lot of scientific discoveries and innovation “the United States (also has)… some of the most backwards proponents of superstitious nonsense in both our electorate and at the highest levels of politics. It is an embarrassment to host laboratories that are at the forefront of scientific research in the same country where presidential candidates are discussing whether Earth is really 6,000 years old as some Bible scholars say, or whether they believe in evolution.” Scientists face a dilemma – if they engage the proponents of creationism and intelligent design in direct debate, they risk giving further credence to anti-evolutionary arguments by inferring that the ideas are worthy of discussion. Authorities such as Richard Dawkins state it is futile to debate with creationists as this lends their ideas credibility. This same process has led to an immunisation “debate” that has resulted in unnecessary deaths. To teach creationism alongside evolution would undermine scientific endeavour and inquisitiveness. The biosciences are integrating our understanding of the natural world from ecosystems to viruses. This understanding and the new opportunities it brings are timely given global threats like climate change. As scientific inquiry is increasingly undermined, ultimately “scientific rationalism (may) be overthrown once and for all.” “Knowledge of history, like knowledge of science, is won with difficulty, and by moral virtues as well as purely intellectual ones. To throw them away dishonours our ancestors and cheats our descendants.” 
[P1] Children can absorb and retain multiple ideas at once. [P2] They can be taught creationism alongside evolution.
Rejecting the premises
[Rejecting P2] Giving credence to creationism fosters anti-scientific ideas which threaten society.
Miller, K. (2008) Only a Theory: Evolution and the Battle for America's Soul. Viking ISBN: 0-670-01883-X Pennock, R.T. (2002) Should Creationism be Taught in the Public Schools?. Science & Education 11, 111–133 (2002). https://doi.org/10.1023/A:1014473504488