Some games lend themselves
to Piggybacking better than others, in large part because some games
are inherently more fun to be an adjunct participant in than others.
In general the hierarchy is consoles over computers, turn-based games
over real-time and thinking games over twitch.
Many of these advantages are obvious. Consoles work better for Piggybacking because it's more comfortable and natural to watch someone playing a game from your couch than it is to sit in a desk chair peeking over their shoulder. Assuming you enjoy both genres equally, turn-based games are more engrossing for a spectator simply because they allow the breathing room needed to make a contribution.
Straight action games are usually spectator friendly only in a very limited "me-centric" way -- you watch, often cringing, leaning from side to side, all the while itching to get your hands on the controller and show whoever is playing how to do it right. As much as you want to help as a spectator, your warnings and suggestions will rarely yield any real benefits and are more likely just to frustrate the primary player.
You can't stick two half-decent Unreal Tournament players together and get one amazing player, but two sub-par logicians working together on an adventure game can easily overcome obstacles where one alone would be adrift.
This is not an indictment of the twitch genres. They are more exciting to watch for most than slower paced titles, but we aren't talking about merely watching; we are talking about ancillary participation. As a rule, the further a game veers away from skill into tactics, strategy, storytelling and/or puzzle solving, the more enjoyable it becomes for the "engaged bystander" because it offers a greater opportunity for involvement -- again, assuming you enjoy all these genres equally well.
A fair and likely question during this discussion is, "Does the parent-child gaming configuration occur frequently enough to warrant devoting development resources to it?"
In light of the negative publicity garnered by the likes of Manhunt 2 and GTA, there are few parents today who aren't curious about the content of games their children play beyond what scant information is available on an advisory sticker, so at the very least I'd say the inclination exists.
But even if the answer to this question is a resounding "yes", a likely follow up would be, "Does targeting parents as an adjunct in children's games translate into better sales?" After all, just because a parent is interested in what their kids are playing doesn't mean they are going to get invested themselves.
This is a nebulous area to track, and hard data simply do not exist (to my knowledge) at this point. Common sense points towards a second, although less emphatic, "yes", but even eking out a timid "perhaps" means immediately we are negotiating time vs. return, so figuring out a way to minimize investment and maximize results seems a prudent thing to do.
When discussing the methods
of creating a Piggyback game, I'm going to focus on the ancillary (spectator)
vector, as this is the defining feature of Piggybacking -- it's what
makes the difference between a game that is agony for a parent to sit
through (regardless of whether or not the child has the time of their
life) and one that is innocuous or even mildly amusing.
There seem to be three broad methods, not including permutations, which are both efficacious and practical for designing a title that encourages parent-child gaming. You can either employ window dressing (and while that term may seem pejorative I don't intend it that way), wrangle mom and dad in with brute force, or level the playing field, making a game that manages to transcend age completely.