This is a question I get asked a lot when one parent withholds children from the other parent. The answer is yes, no, maybe. You technically can take children, if for example they are at school, or in a park, or the children voluntarily come up to you for a cuddle etc. What you cannot do is put yourself in a position when you break the law in order to get the child. For example, you cannot enter into the other parent’s house to get the child, you cannot try and forcibly remove the child from the other parent’s arms.
If the child is left at daycare, then technically you could collect the children from daycare or even a third party’s care. Basically, only the other biological parent can refuse to hand over children.