The Mishna says that a shifcha is believed to testify about the border of the techum shabbos since it is only d'rabonon. Tosafos d.h. afilu, explains based on a gemara in kesubos 28a that a child is believed only after he becomes a gadol to testify about what he witnessed as a child regarding the edge of the techum (Tosafos has two opinions whether he is believed alone or only with another adult). It is clear from Tosafos that although a woman is believed, and even a shifcha, a child is not.
Tosafos d.h. u'techumin, asks why the mishna allows us to believe a woman only because the entire prohibition is d'rabonon, implying that on a d'oraysa a woman isn't believed. Why is this any different that trusting a woman for shechita, nikur and teruma? Tosafos answers that on a d'oraysa a woman is only believed when it is בידה, meaning she is in control of doing a proper shechita, but she is not believed to testify about facts that are out of her control (such as techum, had it been d'oraysa). The rationale seems to be that a woman is not believed to testify about facts because we are concerned that she may not have been meticulous to notice all the details, but when she actually does something and has control of a situation, she is believed to say it was done properly. However, Tosafos points out that the gemara in Pesachim 4b implies that women are only trusted to do bedikas chometz because it is d'rabonon, implying that even though it is בידה she would not be believed if it were d'oraysa (such as a case where there will be bedika without bitul). Tosafos explains that an act like searching for chometz which demands extra diligence and attention, and is a great tircha, a woman is not believed because she will be moreh heter to make assumptions (such as if there is no chometz in one place, there is probably no chometz in another without properly checking). Yet, since it is only d'rabonon, she is believed. In short, on a d'oraysa a woman is believed if its בידה, not if it is out of her control. Even if it is בידה she is only believed if it doesn't demand meticulous attention and isn't a great tircha. However, on a drabonon, if it is בידו even a child is believed (such as bedikas chometz, even though it requires attention), but if it's not בידו such as techum, a woman is believed but not a child.