Being A Future King the odds are he's got reading abilities that are very advanced for his age and its likely that he enjoys reading the kind of childrens stories that most children his age enjoy having people read to them
2 very good examples of books that I used to enjoy people reading to me at that age were The Topsy and Tim Books and The Puddle Lane Books
We could certainly help you by pointing out the zones of the story where we cringe because NPCs would have to behave in a very... difficult to believe way for the backstory to be convincing, but it would really help if you told us who the PCs are and what they are expected to acheive in the campaign, really.
I don't think the point was that the Clown Prince is litterate or not, but rather how a 4 years old, even a smart one, can convince the adults staffed with protecting him (and I guess, ready to protect him
at the cost of their own life) to take him to a place where he is less safe (outside his father's palace, capital city, and even country) without telling the King, and why those bodyguard would devise such an awful plot as to enter the neighbouring country
under a false identity and once there, mostly go run errand for him instead of... doing their duty to PROTECT him. If there is a violent mugging in front of them, bodyguard will take the prince elsewhere asap, not suddenly try to replace the local law enforcing authorities. Tough for the mugged, certainly, but their life depends on them protecting the prince, so it should be the top, if only, priority.
So why didn't they just approach the kid and his guards, explain the situation with the kid and his mum, in which case the guards would quickly get access to A Cleric they trust that will cast Detect Lie to show their telling the truth, then help sort it out and they'd each get a large reward without unknowingly kidnapping a future king
This is a problem, but it's less problematic than explaining why the kid is there in the first place... I think the number of real life kids that managed to convince their kindergarten teacher to take them to another country to visit under a false identity without telling their parents is extremely close to zero.