Now, let me make this clear, this is for toddlers that measure 40"+ with shoes on... on that rare occasion where they actually decide to stand up straight.
Use this advice at your OWN risk, and don't be lazy, ride the ride holding onto you child... don't let a under 18 year old friend/relative ride with your child. If your child is pushing the limit you need to be holding onto him/her or you are just plain stupid. -- As a reference, my 4yr old measures just over 42" now barefoot.. but I will still will not ALLOW him to ride with his 10yr old cousin on Big Thunder Mountain (or any other roller coast- even the Toon Town one) because he will still bounce around and CAN fall out the side because of lap height of the retaining bar. Don't be an idiot, protect your child, just offer him/her the fun that can be had with YOUR arms around them.
Now since Im done with the idiot (disclaimer) wording that seems to be required now (such a sad world, I recommend you do not follow my advice as I will not take responsibility for you not protecting your child).
So, here is the VERY interesting truth of the DLand measurements and how your child that IS tall enough to get in. (And a bit of cheat sheet for those 1-2 inches short).
The measurement sticks at DLand/DCA are not accurate... Ive taken tape measures to them, even argued with the Managers and made them retrieve their OWN (DL issued) tape measures.. and believe me they aren't correct... mostly it involves the tile/concrete/whatever under the measurement device unevenness. The ONLY measurement stick I've found to be accurate is the one OUTSIDE of "Star Tours" (and to the left).
Now, here is the fun... MOST Geox Shoes have a +1 inch heel on them. They are great shoes for children...last long and are usually +0.5 over sized so they last a long time.
The measurement bars on Tower of Terror, Soaring over California, Big Thunder Mountain, Space Mountain are ALL taller than 40 inches. A few of them have placed plastic attachments to TRY and get it closer to the 40 inch mark but its still off because of uneven ground. The measurement bars once inside the ride are the correct height, you just have to get in the door.
Now, how can you get in the door?
There are 3 options.
A GAC (Guest Assistance Card) - This is the MOST difficult option... My son doesnt like to me measured 20-30x in one day, the random CMs grabbing his face and trying to stretch him just plain doesnt work. It takes time @ City Hall as their is no GAC option for height. BUT If you are willing to put in the time and your child is tall enough you can explain that your child cannot be handled by all the random CMs to measure him/her all day to prove the height.
Premium Shoes - For girls they have the Skechers "Pretty Talls", they add 1 inch height, for boys or (even girls) most Geox brand shoes have a 1 inch heel.
Using the +1 inch heel shoes (Skechers - you can take a magic marker to the pink threading on the girls shoes / or Geox), pull the liner out of a old pair of shoes (a smaller size), get your hands on some stiff foam (0.5" think and cut to shape of the shoe heel (about 2-3 inches long) and taper at the end. Place the shoe liner over the top and place inside the shoe. You will HAVE to carry your child as walking will be awkward. This will add 0.6" - 0.8" depending on the shoe liner thickness used... You can add additional height in 0.1" increments with bits of foam however your childs foot must still fit in the shoe and not make them hurt. (Usually can only get 0.1- 0.2 inches extra). Stand up your child at the ending measurement stick and you are good to go.
The biggest trick is feet together, back straight against the pole... worst case MOST Disney employees dont notice your child on tip toes...
Yes I understand those that will disagree with this.. HOWEVER, Im paying $439 a year for my child premium pass... He is capable and WANTS to ride Space Mountain, Tower of Terror, Big Thunder Mountain. He never rides it without my arm around him and as such the fact Disney can't measure isn't my fault. -- On a side note, I usually take him to Tower of Terror early in the AM and tell him to slouch, they will hand out a "special" fast pass good for 6 people for when he is "tall enough" later in the day. Its really SAD I had to come up with these ways to get him on the rides when he measures up since the DLand bars dont measure correctly.