My youngest sister, who is no longer a child, has a global developmental delay that renders her in many ways similar to a high-functioning person with Down Syndrome. Her IQ, when properly tested, shows up around 70 -- she can do arithmetic and can read at an adult level, although it's not enjoyable for her to do so.
In addition, she has a physical issue that prevents her from consuming solid foods, a bit of a challenge when it comes to the host.
My mother worked with the Archdiocese of Detroit through intermediaries and arrived at the following solution: after instruction in the Sacrament to the point at which she could articulate an extremely basic understanding (given that no one can articulate a fully satisfactory understanding), she was deemed prepared to receive the Sacrament. She receives the host in hand and licks it. A fragment of the host contains the Real Presence, so this is deemed sufficient consumption. My mother or father, whomever is accompanying her, then consumes the remainder of the host. (She typically does not consume the wine; she'd be apt to spill it, which, even when done innocently, is problematic.)
With the Archbishop's approval, this arrangement gained ready acceptance. When my parents relocated to Texas, they had someone communicate with someone at their new parish to explain the arrangement, and I suppose the same was relayed to the proper bishop, and no objection was interposed.
Anyway they normally go, the Eucharistic ministers recognize her, so it's no big deal. It has caused some awkwardness when they travel, though.