Even in their resting state, when they appear to do no work, our muscles aren't completely relaxed. In children with low muscle tone, their muscles are more relaxed than normal, so they may appear floppy, or feel soft and a bit squidgy.
This can be a problem when it comes to maintaining a simple posture, such as sitting up straight, simply because they have to work harder at it. My DS2 is demonstrating this quite well at the moment, virtually lying down to eat a sandwich. It also causes him minor problems with walking, since he walks very flat footed with his feet splayed and even though his muscle definition when he's walking is quite good, he gets more physically tired quicker than other 3 year olds typically do.
How old is your dc? Mine had difficulty breastfeeding as couldn't suck hard enough. Head used to suddenly flop forward uptill 8 or 9 months. Late sitting, rolling, walking etc. Easy to push over as not sturdy(but conversely now at 8 she can easily push a grown man over simply because she looks so frail and shoves so unexpectedly) She still has trouble sitting straight and this hinders her writing as well as she likes to supports herself with one hand. That was what low muscle tone meant for my dd.
She cannot crawl or stand or walk. She can sit, very slumped, unaided but tires easily in this position. She cannot maintain a torso position on, for example, a tricycle, she does not have enough tone to pedal (not being able to make legs stiff) If you pick her up it is like lifting a sleeping child, with floppy head - just like a sack o' tatties! (see hoisting thread )