Re: I need advice on tuning a Squier Mini-Strat. (For a 3/4-year-old.)
I got one of those recently. Whilst doing some research, it seems they are fitted with 9-42 strings from the factory !
I've had mine tuned to open G (open E shape up a minor third) with the original strings, and now keep it in F# standard until i fit some 10-46s.
The scale length on these is roughly that of a regular Strat with a capo at the second fret, so F# standard seems to be about right. The open G tuning I used had more tension, perhaps too much for kids (I'd forsee strings getting popped regularly). If you are using the very light strings that came with the guitar, I'd suggest open F or F# (using the open E shape) or open Bb or B, based on the open A shape. I think you could still use these tunings with 10-46.
Or maybe use a set of 11s and use open E or A.
Mine, and the others I saw, have top-loading hardtail bridges. My long-term plan is to modify that to a string-through-body hardtail arrangement, at which point i think 10-46 might work fine for standard tuning. I have encountered advertising that claims they are string-through-body, but none of the ones I saw were.
Out of interest, these guitars are supposed to have bodies made of plywood, and all of the ones I picked up were quite heavy ... except one, which happened to be the first one i picked up. I ended up buying that first one because of it's very light weight, it was just a lot more resonant than the heavier ones. I have no idea what the body of mine is made of, but it certainly can't be plywood, it's just way too light. (Interestingly, all the heavy ones were black, the very light one i bought is red, although i'm sure that's just coincidence).
And of course mine immediately got a set of BKPs stuffed into it ... the Mother's Milk set. The tuners on them are not exactly wonderful, and I'll upgrade those when I re-organise the bridge to get the string-through-body thing happening. That latter operation will increase the string tension and maybe even allow standard tuning using 10-46s.