1. Upon retirement for ordinary disability, a member shall receive a service retirement allowance if he has twenty years or more of creditable service; otherwise he shall receive an ordinary disability retirement allowance which shall be the larger of:

(1) Ninety percent of a service retirement allowance based on his creditable service and calculated the date of disability retirement, or

(2) One-fourth of his average final compensation, except that such allowance shall not exceed ninety percent of the service retirement allowance based on his creditable service and calculated at the date of disability retirement.

2. In addition to whichever of the allowances the member shall receive for retirement for ordinary disability, there shall be paid to him an additional allowance of ten percent of his average final compensation for each unmarried dependent child under the age of eighteen and each unmarried child, regardless of age, who is totally and permanently mentally or physically incapacitated from engaging in gainful employment sufficiently remunerative to support himself, but such additional allowance shall not be paid on more than three children.

3. No benefits under this section shall be paid to a child over eighteen years of age who is totally and permanently mentally or physically disabled and incapacitated if such child is a patient or ward in a publicly supported institution.