"My son needs a small suitcase."
Translation:Mi hijo necesita una maleta pequeña.
You match the gender of the adjective to that of the noun it describes. Since 'maleta' is a 'feminine' noun, the adjective has to be 'feminine', therefore you use 'pequeña' not 'pequeño'. BTW, you need to use a Spanish keyboard (download one) or hold the relevant key on your mobile to get alternative letters. For the 'ñ', hold the 'n'. Remember, 'ñ' is as much a different letter from 'n' as 'm' is.
DL will be trying to teach 'correct' language rather than every form of slang. Using "mijo" instead of "mi hijo"would be the equivalent of using "gonna" instead of "going to". It's something you may be capable of understanding and using well when you're fluent but not when you're starting out learning.
1) you need to match the 'gender' of the adjective to that of the noun, so it needs to have an 'a' not an 'o' at the end because 'maleta' is 'feminine'.
2) you need 'ñ' not 'n' in the spelling. No, this is not something that can be ignored. They are two different letters as much as 'm' and 'n'.