I have it in my yard and I feel that everything in my yard is okay, it looks like (shamrock) clover and yes it looks like creeping woodsorrel. I would just watch your little one and see. Like most things its always best to play it safe..
Have you seen it flower? If it makes little inch long trumpet shaped flowers, then its oxalis. If it makes those little round burr balls that stick to your socks and shoe laces, then its burr clover. Oxalis isn't toxic, but because its so high in oxalates it binds up the calcium, making it unusable for your little growing tortoise. If they ate enough of it, on a regular basis, it could cause MBD. I think the burr clover is fine as part of a varied diet. Regular clover, either white or red, is a great tortoise food.
Even without flowers, you can tell by the taste. Chew up one leaf. Sorrel (oxalis) is pleasantly tart, not as sour as a lemon. Clover is.... neutral, disappointingly not tart.