The gemara 31b says a cheiresh, shoteh, and child cannot be used to establish an eiruv techumin. Rashi explains that to establish an eiruv techumin one has to be able to make an acquisition since the concept of eiruv techumin is to be ko'neh shevisa. Therefore, all those who cannot create kinyanim cannot be ko'neh shevisa. Tosafos disagrees and holds that eiruv techumin is like eiruv chatzeiros in the sense that no acquisition needs to be made. By simply having the food placed at that location with the intent of the owner to serve as an eiruv, it automatically goes into effect. The problem with the cheiresh, shoteh v'katan is simply that they don't have a sufficient level of trust to believe that it was done. The reason why we demand a higher level of trust for eiruv techumin than for eiruv chatzeiros is that eiruv techumin is based on an esmachta, raising its status to be similar to a d'oraysa.
The gemara on 32a has discussion whether one can trust that a shliach fulfilled his commitment without him returning to verify that it was done. All agree that Eiruv Techumin which although an esmachta is only d'rabonon, therefore we can rely that the agent fulfilled his commitment, but on something which is d'oraysa (meaning that if it were not done, it would be a d'oraysa violation) we have a machlokes Rav Nachman and Rav Sheisheis whether or not we can trust that it was done. In the context of the gemara the gemara tries to prove from a case of separating ma'aser on figs which is only d'rabonon (not from the dagan, tirosh, v'yitzhar), implying that it is considered d'oraysa. Tosafos asks this question and explains that although it is not d'oraysa, it is treated like a d'oraysa in that we don't trust that the shliach fulfilled his commitment since it is "עיקרו דאורייתא" and sourced in the Torah.
Techumin is considered like a d'oraysa in the context of not trusting a child, but is considered a d'rabonon in the context of being able to rely on a shliach, yet, ma'aser on fruits which is only d'rabonon is considered a d'oraysa even for the purpose of trusting a shliach.