The standard view in Linguistics is that there are no easy or hard languages for either children L1 learners or older and adult L2 learners. It is also said that all languages are equally complex and no language is more simple or more complex than any other. On its face, this seems preposterous, especially for L2 learners. Linguists say that it all depends on what L1 you are coming from.
There are anecdotal reports that Navajo children have a hard time learning Navajo as compared to children learning other languages, but Navajo kids definitely learn the language.
Reportedly, Nambikwara children do not pick up the language fully until age 10 or so, one of the latest recorded ages for full competence. Nambikwara is sometimes said to be the hardest language on Earth to learn, but it has some competition.
Adding weight to the commonly held belief that Arabic is hard to learn is research done in Germany in 2005 which showed that Turkish children learn their language at age 2-3, German children at age 4-5, but Arabic kids did not get Arabic until age 12.
This implies that from easiest to hardest, it is Turkish -> German -> Arabic.
Italian is still easier to learn than French, for evidence see the research that shows Italian children learning to write Italian properly by age 6, 6-7 years ahead of French children. So at least in terms of writing, it is much easier to learn to write Italian than it is to learn to write French.
Careful studies have shown that English-speaking children take longer to read than children speaking other languages (Finnish, Greek and various Romance and other Germanic languages) due to the difficulty of the spelling system. Romance languages were easier to read than Germanic ones. So in terms of learning to read, from easiest to hardest, it would be Romance languages -> Finnish/Greek -> Germanic languages except English -> English.
Suggesting that Danish may be harder to learn than Swedish or Norwegian, it’s said that Danish children speak later than Swedish or Norwegian children. One study comparing Danish children to Croatian tots found that the Croat children had learned over twice as many words by 15 months as the Danes. According to the study:
The University of Southern Denmark study shows that at 15 months, the average Danish toddler has mastered just 80 words, whereas a Croatian tot of the same age has a vocabulary of up to 200 terms.
[...] According to the study, the primary reason Danish children lag behind in language comprehension is because single words are difficult to extract from Danish’s slurring together of words in sentences. Danish is also one of the languages with the most vowel sounds, which leads to a ‘mushier’ pronunciation of words in everyday conversation.
Therefore, Danish is harder to learn to speak than Croatian, Norwegian or Swedish. From easiest to hardest to learn to speak, it is Norwegian/Swedish -> Danish and Croatian -> Danish.
Russian is harder to learn than English. We know this because Russian children take longer to learn their language than English speaking children do. The reason given was that Russian words tended to be longer, but there may be other reasons. So from easier to harder to speak, it is Russian -> English.
It is said English-speaking children reach full adult competency in the language (reading, writing, speaking, spelling) at age 12. Polish children do not reach this milestone until age 16. So from easier to harder, it would be Russian -> Polish -> English.
