Statistically speaking, males are overwhelmingly more likely to sexually abuse children. That's an undeniable fact.
Using this large study of Australian men, nearly 5.7% of surveyed men would have sexual contact with a child between 12 to 14 years if no one would find out, 4.6% would have sexual contact with a child between 10 to 12 years if no one would find out, and 4.0% would have sexual contact with a child younger than 10 years if no one would find out.
Around 1 in 10 (9.4 per cent) Australian men have sexually offended against children, and men with sexual feelings towards children are - of course - more likely to work with children.
Meanwhile, while females can and do perpetrate sexual abuse against children, the percentages are so much lower that it's been impossible for me to find any study that examines what percentage of females have sexual feelings towards children.
Female offenders also seem to target different age groups (overwhelmingly pubescent males, one large-ish study of female sex offenders appears to show), and have very different psychological profiles and motivations to male offenders. But of convicted sexual offenders (against all ages), females make up 2% on average, although it's estimated from one victimisation survey that female offenders may make up as much as 12%.
Basically, males are far, far, far more likely to present a danger to children in terms of sexual abuse. If you consider early childhood care to be a necessary, calculated risk (like driving a car is), then allowing male nursery workers is, imo, like driving with your seatbelt unbuckled, or while slightly over the limit in terms of blood alcohol levels - it's likely that nothing will happen, but you're introducing an unnecessary, major risk factor that could be avoided.