title_body: Which features make a language easier to learn? Which features make a language easier to learn for children learning a constructed language as L1? Are children learning a language with these features able to learn the language faster than a natural, irregular language without some of these features?
upvoted_answer: Difficult question. I think regularity would speed up learning, as children during language learning overgeneralise (see experiments with English past tense endings). Thus instead of learning the correct exceptions at a later stage the corresponding feature would have been learned earlier.
  
  There is a programme for teaching Esperanto as a first foreign language, as it can be used to increase second language awareness and is supposed to make learning a natural foreign language easier. You don't have to worry about exceptions, for one thing, so you can teach the concept without having to hedge your explanations by listing all the cases where it's different.
  
  My personal view on 'difficulty' of different languages is that all languages are about equally difficult. Some languages are easier/simpler regarding some features (English inflectional morphology), but make up for that in others (vast number of near-synonyms with different distributional characteristics in English). Toki Pona is a very simple language (120 words, virtually no syntactic complexity), so you can learn it almost in a day, but it is very hard to express complicated narratives, and an equivalent text in a more complex language would be far more concise. 
  
  Learning Esperanto is reasonably easy, but then you look at how people actually use it in everyday interactions, and you find that it is more difficult than you imagined: there are a lot of the smaller particles that make it tricky to understand for a beginner. But the 'early' sentences composed when the language was still fairly new are a lot simpler. However, they were not suitable for encoding all the pragmatic and other information people need for an everyday language.
  
  So, there are undoubtedly features that make it easier for children to learn a language as L1, but the question is "what for?" It's a bit like learning Lisp programming: understanding the syntax takes literally ten minutes, but that does not mean you can write useful programs in it after that. Languages are more than the sum of syntax and vocabulary, and if there was a language that was easier to learn and use than other languages, and had equal expressive power, then we would all be speaking it already.