It's not ideal for such a young plant, for one that's a few weeks old it wouldn't be too bad. Place the thermometer on the pot and see if it goes up at all. The floor can be colder than the top of the plant. Aquarium thermometers with remote sensors are really good and can be as little as £2 to buy. You can hang them so the sensor is just above the plant so it gives a more accurate reading of what the plant is being subjected to