I shudder at the thought of PreK's holding a $400 handheld device.

I taught PreK5th for 10+ years in two different church computer labs using desktops and have had no problem with PreK's -other than trying to get the kids away from the computers!  

If "viewing height" is an issue....  I know of several churches that used a picnic bench so the little kids could sit up a bit higher and slide together. Another found that adjustable height office chairs made it easy for kids to get to the mouse. If you have "computer furniture" that may be part of the problem. Standard long tables are much preferred in multi-age, multi-use lab. I know many PreK-K labs solve their lab config and software issuse by hooking up an older XP laptop to an LCD projector. The little kids love that.

Software for pads is going to be an issue. There are (very) few Bible apps for PreK and none of them very inspiring, especially if you're laying out serious cash for pads. 

In a church setting, I also don't think it's a sin to RESTRICT the lab to older kids if you have to choose between older and younger for whatever reason, or are facing a budget issue.   It is our older children who need the attraction of computers MUCH MORE than our younger ones --who still enjoy crayons.  So I'm not uncomfortable suggesting to anyone that if you can't make it work for PreK's, don't sweat it, and don't break the bank trying. Instead, give the lab time to the older kids who really need it, where the bulk of the software exists, and where the equipment setup is a lot easier and cheaper.

<>< Neil