Paul is correct in the statement that fuses of higher voltage rating can generally be substituted for those of lower voltage rating.
It’s possible that the 5A/6V specification is not referring to the spec for the fuse itself, but to the expected maximum values of the power that it needs to carry–this would be an atypical and ill-advised way to specify a fuse, but seems likely to be the case here. The other plausible explanation is that it’s a misprint meant to refer to a 5A/600V fuse, but such a spec would be more suited to industrial robots, not toy robots.
More to the point, it would appear that the fuse pictured may actually be rated for 7.5A, based on the markings. This would be consistent with the first theory above, in which case the 031107.5M or something similar might be a suitable option. A 5A fuse carrying 5A will nuisance-blow in relatively short order, so a 7.5A fuse in a circuit expected to carry 5A under normal conditions would be a better choice. And 6V corresponds to 4x alkaline cells or one 6v lead-acid battery, which seems a lot more likely for the application in question.
See this resources if you’re interested in further info on what the various selection parameters for fuse products mean.