Toy encryption system that provides "hints"
You can try NSA's CryptoKids page (archived here). It's been around for some years and the ciphers have varying difficulty so you might at least get some ideas.

Alternatively you can use plain XOR encryption with a twist: Encrypt multiple consecutive letters with the same character. For example, if $|m|=n$ then $|k|=n/4$ and do $c_i,...c_{i+4}=k_i \oplus m_i,...,m_{i+4}$. That should give good feedback with every correct guess.