My guess is that your body is stiffer in the morning and looser in the evening, with the result that when you take Balasana (Child’s Pose) in the evening, your body releases more freely over your legs and into the flow of gravity. This translates into greater pressure on your forehead as it presses into the mat in front of you.
Child’s Pose is a posture that we usually use to rest or relax, so it should be a comfortable pose. You might try propping your forehead up, either on your hands or on a block or blanket, and see if this relieves the feeling of pressure. If it doesn’t, I recommend substituting a comfortable, cross-legged position such as Sukasana (Easy Pose), or kneeling with your buttocks supported on a block in Virasana (Hero Pose) when this pose comes up during your evening practice.
As to the issue of whether some poses are better in the morning or afternoon, aside from the general distinction between heating/energizing poses such as Sun Salutes and backbends, which tend to be more beneficial in the morning, and cooling poses such as forward bends and certain inversions, which are more likely to calm one down in the evening, it really depends on the person. If, for example, you are naturally a more flexible or “bendy” person, there is a case to made for practicing in the morning when you are a little bit tighter, and therefore less likely to injure yourself by over-stretching. If you are naturally on the stiffer end of the spectrum, you may feel more limber if you practice in the latter part of the day, after you have been moving around for a while. Different factors come into play for everyone, so I recommend observing the effects of your practice and of specific postures at different times of the day (as you did with Child’s Pose). Then make your determination afterward, based on the way you feel both physically and energetically.