I'm happy with the idea of grid kids, as long as there is a selection process based on merit at the proposed role rather than lottery or skill in karting (national sporting authorities will get both options, provided they select from among their under-16 karters). Some children (especially teenagers, but even some people a little younger) are perfectly capable of standing around holding a sign, and could make some useful paddock connections that way. Others, as Scott points out so well, are just an aggravation waiting to happen in such a role. Thankfully, F1 has been sensible enough to require parents (or, presumably, whichever adult(s) is/are accompanying each child) to join the children on the grid, which is likely to put a lid on minor examples of overenthusiastic disobedience. (For example, don't expect to see the grid itself turn into an autograph session, for this reason).
Hopefully, the fact that it is families/groups going onto the grid, not just single people (of any age), will deter race organisers from making the children wear anything egregiously awful (either in the scantily-clad sense, or any other possible interpretation of that sentence). I also assume that the rules on the back of F1 race tickets, which prevents anyone from objecting if they are recorded and broadcast, would apply equally to the grid as it does everywhere else in trackside venues - preventing any possible objection from parents to their young ones being photographed.
I will also say: only karters from the nation hosting the race (or, probably in the case of Monaco, these plus karters from nearby not doing the duty at their actual home Grand Prix) can be selected. This means that nobody under the age of 8 could possibly be selected - nor anyone who is being accompanied by a single adult plus a sibling below the age of 8.