Children are a lot smarter than people give them credit for in a lot of ways. For one, the mind of a child is very malleable, or, to use the technical term, has a great deal of neural plasticity. This is a fancy way to say they can learn complex concepts really well if the concept is properly explained and taught. This includes languages.
Children are in fact very good at learning languages if they are taught properly, at the same time, children are still kids and tend to make funny mistakes while doing pretty much anything. This is especially true when it comes to a language as convoluted as English.
The most common issue when teaching English to young children is the fact that they may be easily capable of learning a language, their brains are still young, tongues small, and teeth tiny, so proper pronunciation is a major issue. This is why tongue twisters are a common teaching device for English. Besides being a good way for kids to learn proper enunciation, tongue twisters are fun because it’s hilarious to watch a child try and say “she sells seashells by the seashore” and screw it up over and over again.
Another common issue for kids is large and complex words.
Mispronunciations of those words are common and can even be an issue for adults. Similarity, curriculum, and other multisyllabic words – such as multisyllabic- can tongue tied even adults. This is one reason when kids start learning English it’s usually smaller words first. Hence why, along with tongue twisters, another common way to teach kids a language is with visual comparison, or, in other words, flash cards.
Flash cards are a tried and true method of learning, but they’re not for everyone. Even so, it’s hard not to think of a stack of cards with a puppy or cat and having a kid spell out the word. It’s also not uncommon for very young children to make associations based on what they know. This is how we get terms like trash panda and raccoon dog; a tanuki looks kind of like a dog-like raccoon, and move on from there. Wolves are big puppies, tigers and big kitties, and so on and so forth. People will make comparisons based on what they understand, and kids have a very small pool to draw from.
Mixing up similar sounding words is another common such issue. Very young children will likely mix up consonants or even drop them to say simpler sounding word-like sounds. The Nickelodeon show Rugrats provided excellent examples of this, as the babies would confuse explanations and words to conform to their limited understanding of the world. For example, simple concepts such as hospitals or traffic tickets held different meanings for them. As for words, “aminals” and other mix-ups were common mispronunciations. For Better or For Worse did this as well, with small children saying “an” instead of “and” all the time.
Children can learn very rapidly and more deeply than adults give them credit for, but at the end of the day they are still kids. English is a difficult language and can take time to understand all the various nuances. The use of word association games, crossword puzzles, and other such tricks can help, but even so kids will make mistakes, and those mistakes are often pretty funny. Kids mispronouncing words or using other words to convey what they mean is part of growing up, and as adults we get to experience the fun of learning along with them. English might be a difficult language, but is also a hilarious one.
You can view the list of stop words for English language at page English stop words.