In point of fact, what makes the letter difficult to read is not so much the spelling but the lack of spaces between words. Youdontrealizehowimportantspacingisuntilitsmissing. But I don't mind. It worked for James Joyce.
So here is my letter, which he wrote all by himself:

And it reads:
Mommy,
I love you and I hope that we will (something something) Christmas.
Primo
My baby wrote me a letter. Its been making me smile all day. My baby wrote me a letter.
PS. Anyone who can decipher wha words lie between will ("wl") and Christmas ("Qaestmeas") gets a prize. Of course I'll have to rely on Primo as the judge since he's the only one who knows the real answer, but I'm sure he'll be fair. He's five after all.