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No, people won't feel like its a children's book. As other people said, it is relatively common in adult literature. But know that it is unlikely to actually happen unless you are well-established....
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/2611 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
No, people won't feel like its a children's book. As other people said, it is relatively common in adult literature. But know that it is unlikely to actually happen unless you are well-established. It increases the cost of printing tremendously, since images have a much higher printing quality necessary to "look good" than words do. Typically only with "guaranteed successes" like Brandon Sanderson's The Way of Kings do publishers take the risk of including decent illustrations. If it is not possible, an alternative is posting them in full glory on your web page as an extra, and referencing the page in the book. Invested fans will seek out that material.