A wish for William Tyrrell's family on his fourth birthday.

William Tyrrell is four years old. And we’re thinking about his family, today more than ever.

It’s been nine months since little William Tyrrell was last seen.

The little boy was playing in his grandmother’s yard on the NSW Mid-North Coast in his favourite Spiderman suit when he disappeared on September 12 last year.

His devastated family has not seen him since; they have not looked into his attentive brown eyes, kissed his chubby cheeks or patted that fine, silky little-kid head of hair for 41 whole weeks.

All that time they have waited, hearts in chest, as police scoured bushland and coastal properties for traces of the boy they love. They have, we imagine, wept and cursed and screamed through the series of painful, fruitless searches: holding their breath as a water tank was drained to search for clues, hearts dropping as a Spiderman toy was found in the van of a local man, hoping against hope that links to a possible paedophile link were not borne out.

And today, surely more than any other day, they have wondered: Who knows the truth about our little boy?

Today is William Tyrrell’s fourth birthday. To mark the bittersweet occasion, some Australians are switching their Facebook profile picture to photos of William’s unmistakeable grin, an image in which he poses like any happy Australian boy by the sea as he grips a sippy cup.

It’s a cheeky grin that, by now, so many of us know by heart — and we really shouldn’t.

Listen to The Quicky debrief on the truth about William Tyrrell’s parents, and what happened after the three-year-old’s disappearance. Post continues below.

We really shouldn’t know who William Tyrrell is, because he should be at kindy today playing birthday games with his friends rather than frozen in time, splashed across the pages of every media outlet in the country.

He should be looking forward to a big kiss from his mum and dad, a cake covered with colourful designs and an accidentally-too-late bedtime tonight as he tosses and turns with excitement at his new toys.

Today his grieving family will instead mark the day by making a cake to share with friends and family. And even though there’s no small, excited child present to blow out its candles, they have not given up hope. Instead, they’ve issued a plea for Australians to stand by them as they keep the search for their boy alive.

“Please don’t give up on our boy. Not a day, hour, or minute goes by without there being a reminder of him,” they said today, in a letter read out by Bravehearts CEO Hetty Johnson.

“This is a pure living nightmare. How could we be involved in this simply astonishing situation let alone still without an answer?

“Someone knows something. Someone always knows something.”

Today, as NSW police renew their public appeal for information on his disappearance, they will be thinking of the little boy they love.

So today, we are united in our wishes for William Tyrrell’s family: Wishes that little William will be found. Wishes that the innocent child — four years old, a’ big boy’ now — will be back in their arms again soon.

Wishes that, at the very least, police will turn up at the family’s door with some news, any news. Wishes that whoever has been keeping the secret of William’s whereabouts to themselves is brought to justice.

Or perhaps just wishes that, at the very least, this is the last birthday William’s family is left wondering the truth about their little boy’s whereabouts.

Some social media tributes to William Tyrrell on his fourth birthday, via Facebook group William Tyrrell, Where Are You?

Have you seen William Tyrrell? Police are urging anyone with information in relation to this incident to contact Crime Stoppers on 1800 333 000 or use the Crime Stoppers online reporting page here. You can also donate to the Where’s William campaign here.