My understanding is that even if you have decent UVB lighting in your viv, it is still important that baby bearded dragons get the necessary amount of vitamin D3 for them to properly grow and develop.
What I do is I
very lightly coat my crickets with calcium&vitamin D3, and a multivitamin powder once a day before feeding time.
If you do this, you will not have to worry about giving your baby beardie too much vitamin D3. As they get older, however, you will need to give them less vitamins. When they are full adults, you would only need to coat their insects with calcium and vitamins 1-2 times a week.
But there are some people who have reported not using D3 in their calcium because of the
UVB light.
I just feel like if I make sure to not over do the calcium w. D3 when I do use it, then I don't see an issue with having D3 in the calcium.