Essentially to make a balaclava you need to measure from just before and above the ear round to the other side (around the widest part of the head at the temples). Then take some yarn and needles and knit a stocking stitch 'swatch' of about 10x10cm. Measure how many stitches you have to around 2cm and then work out how many stitches you would need to fit the measurement you have taken of the head.
The on smaller needles than you have knitted the swatch with (e.g. if you knitted a swatch with 4.5mms you would need to cast on with 4mms) cast on the number of stitches you need plus around 20/22 more if you are using aran weight - you will need more than this if you are using a lighter weight of yarn. You could measure across the base of the child's neck if you want a rough idea of how many extra to add, but remember ribbing is stretchy.
Then double rib for a few centimetres (as much as you like really to cover the child's neck). Bind off one at the beginning of next two rows (these will be used in the seam allowance). In aran weight for a child of 2/3 I ribbed 15 rows when doing one for the pattern - ideally I think there should be more.
Put 10 stitches (or divide the extra you added) at each end on holders and then swap to the needles you knitted the swatch with and start stocking stitch. Continue until you reach the top of the child's head.
Then you need to start short rows. Divide the number of stitches you have into 3 'panels'. If the number you have isn't divisible by three, add the extra stitches to the central panel.
To short row you knit or purl across the first two 'panels', skpo or p2tog (depending on whether you're on the right or wrong side) and then turn, sl 1, knit/purl back along the central panel, either skpo or p2tog, then turn and do the same. Essentially you are picking up a stitch from the outer 'panels' at each end of the row by skpo or p2tog. As you do this the top of the balaclava will fold over.
Once you only have the central panel remaining you need to put these stitches on a holder and break the yarn.
Then sew the front ribbing together and then, using circulars or dpns, pick up stitches around the face, ribbing the stitches on holders.
I always end up frogging this back on first attempt *sigh*, I either pick up too little or too many, but as you're only ribbing a few rows this isn't any great hardship. Rib in the round for a good few rows and then bind off.
You could try using the pattern on NNN http://ninnynoodlenoo.com/content.php?content_id=1019 as it may fit an 8month old - one way to do it would be to cast on the number of stitches stated in aran, rib a few rows and then using a measuring tape measure the ribbing and then measure the child. That should give you an idea of whether or not it would be baggy and whether you might have to cut out a few stitches.
They don't take that long to knit up (good job really with small children running around and I rather lack patience, so am notoriously bad at finishing anything like adult size jumpers, etc.)
I'm rather useless at writing out patterns, I tend to make things from my head now, with the odd reference to a book for a lace or edging pattern - if you're only making small things this isn't so much of a problem, I wouldn't attempt a cardi or similar for me with the same gungho approach, but for little things like this (and dolly clothes) mistakes don't take too long to rectify.
If you're in the South Yorkshire area I'd be happy to offer a tea/coffee and some knitting support