I had to put DS1 in short trousers yesterday. He sat on the floor put his shoes and there was a big flash of red underpants. The seam had gone from the zip round to the back. It was 8.40am so I told him he would just have to sit with his legs together all day so as not to scare anybody. He was horrified and said he had to sit crossed legged, cos his teacher said so!
No choice since the only other pair he had were too small. Thankfully not the whole expanse of ankle was on show.
Mustrunmore - am impressed your 4 y.o. can get into an 18mth old's trousers at all. My 4 y.o can't - rugby player thighs, bless him.