It really comes down to someone saying "I will know it when I see it." Heather has a good rule that I think should keep you out of trouble. All I can say is, if you are uncomfortable with it then don't do it. Listen to your instincts. If you do agree, make sure the adult is a legal gardian and that you get written concent prior to the photo shoot. You may beat the rap but you may nay beat the ride.
My neighbor's wife has pictures of their children in situations like the bath tub, but the soap bubbles obscure from visibility any "objectionable" parts.
From the Texas Penal Code:
http://www.statutes.legis.state.tx.u...2085.53404.htm
Sec. 21.15. IMPROPER PHOTOGRAPHY OR VISUAL RECORDING. (a) In this section, "promote" has the meaning assigned by Section 43.21.
(b) A person commits an offense if the person:
(1) photographs or by videotape or other electronic means records, broadcasts, or transmits a visual image of another at a location that is not a bathroom or private dressing room:
(A) without the other person's consent; and
(B) with intent to arouse or gratify the sexual desire of any person;
(2) photographs or by videotape or other electronic means records, broadcasts, or transmits a visual image of another at a location that is a bathroom or private dressing room:
(A) without the other person's consent; and
(B) with intent to:
(i) invade the privacy of the other person; or
(ii) arouse or gratify the sexual desire of any person; or
(3) knowing the character and content of the photograph, recording, broadcast, or transmission, promotes a photograph, recording, broadcast, or transmission described by Subdivision (1) or (2).
(c) An offense under this section is a state jail felony.
(d) If conduct that constitutes an offense under this section also constitutes an offense under any other law, the actor may be prosecuted under this section or the other law.
(e) For purposes of Subsection (b)(2), a sign or signs posted indicating that the person is being photographed or that a visual image of the person is being recorded, broadcast, or transmitted is not sufficient to establish the person's consent under that subdivision.