As stated in the OP, English-language food guides do not mention mushrooms as foods to avoid (example from the UK's NHS). This doesn't necessarily mean that mushrooms are good, since mushrooms may not constitute an important enough food source in the UK, though if they were very dangerous, I would expect to find them listed.
It's not quite clear to me what "Difficult to digest" really means - if the fear is that the mushrooms pass mostly undigested, then mushrooms are not a big problem. Quite a few foods seem to pass through the system hardly digested, for example peas, when the kids cannot chew very well yet, and when their digestive system is still maturing. The only possible concern is that if mushrooms make up an important fraction of the diet, children do not get enough calories (in fact, mushrooms are supposed to help you lose weight, something you're usually trying to avoid in small children).
However, if you understand "difficult to digest" as leading to "feelings of being unwell", the difficult-to-digest mushrooms may actually be slightly toxic. Toxicity is measured in terms of amount per body mass, so the same amount of mushrooms that leads to a slight indigestion (or not even) in an adult could have more severe consequences in a small child. Thus, if you know that specific kinds of mushrooms make you unwell, you shouldn't be giving them to a kid.
In sum, as long as you get properly sourced properly cooked (and reasonably radiation-free) mushrooms that don't make you feel unwell, and as long as these mushrooms aren't making up a large portion of the kids diet, all should be fine.