Brief Answer:
A thorough investigation is required.

Detailed Answer:
Hello and thank you for choosing 'Ask a Doctor' service for your query dear,
I have seen the details given by you and I could appreciate your concerns,

Very likely the crack is not a simple one it could be an anal fissure and fistula.

A fissure/Fistula could appear like a small ridge/external crack to normal eyes but is usually connected internally to glands around the anal/ rectal canal.

History of complete immobilization again supports the diagnosis.
However in medical science first and foremost rule is to examine the patient.
It is recommended to book an appointment with an MD/General surgeon.

The doctor would examine the area and would likely do proctoscopy(would try to visualize things inside the anus by a small device) and finger testing.

Thereafter either a Fistulogram or MRI would be done to track the route of crack(internal).
Sometimes it gets infected and causes the formation of a large amount of pus inside which may slowly cause severe infection.

Once diagnosed Video-assisted anal fistula treatment (VAAFT) would be done. In your daughter, it should be the treatment of choice as it is a novel minimally invasive and sphincter-saving technique(chances to lose control over anal sphincter are minimal).

As of now use a laxative and stool softener to her schedule so that stools could be passed without strain.
Lactulose is a safe and good one and is over the counter.
10 to 30ml could be used once daily at night as per patient convenience.
Apply any antiseptic ointment like Mupirocin thrice daily over the area.

A broad-spectrum antibiotic like Cefixime 200mg (prescription may e required by your local physician) can be given along with a combination of (Trypsin and chymotrypsin) Chymoral forte twice daily to prevent infection and heal up the crack.
Vitamin C tablet should also be taken daily.
Consult your doctor and follow up with what s/he suggests.

Hope I was helpful,
Thank you

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