单纯是个褒义词，不管你形容的对象是谁。 天真用来形容孩子是ok的，但是形容成年人的话暗含了一种“你这个人一点都不成熟”的负面色彩，比如“你太天真了”就不是在夸人

天真多指人的思想没有或者很少受后天影响而保留的原生状态。一般多用于小孩。 单纯多表示思想简单，也指受后天环境的影响小。一般多用于有了一定社会活动的年轻人。 后天环境指人受教育及社会中各种各样的影响。

They are similar，单纯 means simple and pure literally. 天真 is often used to describe children. 单纯 describe a person who is simple and has no evil intention. While 天真 describe a person who is naive and maybe easy to cheat, but only a little bit. And in my opinion, 单纯 is used to describe young girls between 12-20s years old while 天真 is used to describe toddlers.