Rose that's completely normal and developmentally appropriate for a child before puberty. Don't worry If it were me i'd use distraction or avoidance if you don't like it - don't tell them NOT to do it, just give them a more-fun bath toy, or bath them seperately or whatever. Education is important but so is normal development and a curiousity about genitals is a very normal part of children growing up.

Here's a list of age-appropriate behaviours common ones are cool, uncommon ones may be red flags for abuse:

PRESCHOOL AGE (0 to 5 yrs.)
Common: Sexual language relating to differences in body parts, bathroom talk, pregnancy and birth. Masturbation at home and in public. Showing and looking at private body parts.

Uncommon: Discussion of specific sexual acts or explicit sexual language. Adult-like sexual contact with other children.

SCHOOL-AGE (6-12 years)
This group may include both pre-pubescent children and children who have already entered puberty, when hormonal changes are likely to trigger an increase in sexual awareness and interest.

Pre-pubescent children
Common: Questions about relationships and sexual behavior, menstruation and pregnancy. Experimentation with same-age children, often during games, kissing, touching, exhibitionism and role-playing. Masturbation in private.

Uncommon: Adult-like sexual interactions, discussing specific sexual acts, masturbating in public.

After puberty begins
Common: Increased curiosity about sexual materials and information, questions about relationships and sexual behavior, using sexual words and discussing sexual acts, particularly with peers. Increased experimenting including open-mouthed kissing, body-rubbing, fondling. Masturbating in private.

Uncommon: Regular adult-like sexual behavior, including oral/genital contact and intercourse; masturbating in public.

ADOLESCENCE (13 to 16)
Common: Questions about decision making, social relationships, and sexual customs; masturbation in private; experimenting between adolescents of the same age, including open-mouthed kissing, fondling and body rubbing, oral/genital contact. Voyeuristic behaviors are common in this age group. Intercourse occurs among approximately one third of children in this age group.

Uncommon: Masturbating in public. Sexual interest directed toward much younger children.
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