Definitions:
"Genital autonomy" - the right to decide what happens to one's genitals, even to the point of irreversibly altering them.
"Sexual autonomy" - the right to decide to enter into sexual relationships with other people.

In many countries, informed patient consent for gender-affirming medical treatments (for the purpose of this question, these are pharmacological and surgical treatments) can be given by patients over the age of 18 (e.g. Portugal, Austria, Italy, US), over the age of 16 (Netherlands, Republic of Ireland, Malta), or corresponding to the general rules for medical informed consent based on invasiveness/long-term effects of the procedure and a patient's mental maturity on a case-by-case basis (UK, Spain, France, Germany, etc, EU source here). For younger patients, parental consent is required.
This seems to be a generally reasonable spectrum, overall in line with the respective societal norms in terms of the autonomy granted to children and adolescents. So far so unproblematic.

Suppose Bob wants to argue that gender-affirming treatments should be available even earlier at the request of the child in question.

For now, let's consider the extreme position of genital-altering surgery for pre-pubescent children. Can Bob reasonably hold this view without simultaneously holding the view that pre-pubescent children can in fact validly consent to sexual acts with adults?

Consider:

Premise 1: A pre-pubescent minor can give valid consent to have genital-altering surgery.
Premise 2: Actions with more severe consequences, especially when irreversible, require a higher capacity to consent than actions with less severe consequences in order for that consent to be valid.
Premise 3: The consequences of genital-altering surgery are more severe than the consequences of nonviolent sexual intercourse with an adult.

Conclusion 1: A pre-pubescent minor has a higher capacity to consent to nonviolent sexual intercourse with an adult than is required for that consent to be valid.
It follows that a pre-pubescent minor can validly consent to sexual acts with an adult.

Premise 1 is taken as true for the sake of argument.
Premise 2 seems inarguable, it certainly forms the basis for most modern applications of informed consent in medicine, law, and interpersonal contact.
Premise 3 might be arguable as follows:

• Genital-altering surgery will have positive future effects, whereas sexual intercourse with an adult may have negative future (emotional) effects. That is why they must be treated differently. Bob's objection would be that the expectation of positive future effects would have to go back to the child's state of mind and be weighed by the child as part of giving valid consent. After all, if the child wrongly decides in favor of surgery, the future effects may turn out to be disastrous. Why should the minor be able to weigh the possible positive long-term effects of surgery, but be unable to weigh the possible negative long-term effects of having sex with an adult? The objection seems self-defeating.
• A pre-pubescent minor generally does not understand what it means to have sex, but they do understand their felt gender. Bob would object that the surgery's impact on the individual's future sex life is enormous. If the child does not understand the nature of sex (inarguably a cornerstone of the human experience), then they are unable to give valid (i.e. informed) consent to a procedure that fundamentally changes or even prevents their future opportunities to experience it. But since we assume, arguendo, that children can give valid consent to such procedures, we must concede that such children have a sufficient understanding of sex to factor it into giving valid consent.
• All sexual intercourse of children with adults is by its very nature violent and forced (e.g. because of a power differential) and thus has more severe consequences than even irreversible surgery. Consent becomes irrelevant, analogous to the fact that one can not legally consent to be murdered. This would seem to be strong, non-obvious categorical statement and would need to be supported. It also seems to be a non-sequitur. Could a similar argument be made for parents "strongly supporting" that a child receive gender-affirming therapy?

To summarize Bob's point: If the child can consent to having their breasts cut off, then the child can consent to having them fondled.

This question does not exist in a vacuum, but rather in the context of consevative and far-right political groups associating - implicitly or explicitly - the gender-affirming/trans movement with the practice of grooming and, more generally, paedophile intent.