How to get evidence of child abuse/neglect?

my husband and I met with a lawyer today to get advice on what we should/shouldn’t do about my nieces/nephews who are being abused, and he told us we needed more hard evidence to make a case if we want to be SURE to win.

I’m going to be vague just in case, the kids father died a while back suddenly, their mom had been cheating with a man she met while working at a jail for a short period of time (side note: I cannot find his records or any evidence he even exists! maybe using his middle name or fake name??) anyways, they moved a couple hours away and now lives with the boyfriend, they are not being taken care of, mentally, physically, medically, emotionally, they aren’t allowed to even talk about their dad at home.

the thing is, we don’t have much proof, we have some pictures of them in the car while it’s moving with no seat belts, a video of the younger kid hitting, scratching, pulling hair, punching themself and talking very badly about themself and a picture of how disgusting their last home was and that’s all.

I’m not sure what I can use in court or how to get evidence of anything that we have witnessed! how can I do this without the mom catching on? what do I need to be asking/looking for? and who would be the best person to talk to about this? if we contact CPS will they automatically open a case?

edit to add a bit more info!

-the youngest (almost 6) is still not potty trained, he can’t count past 10 and can’t sing his ABCs confidently, and he is not close to being potty trained like at all, he went though half a box of pull ups in less than 48 hours!

these are my main concerns but there’s a bunch of small things that I don’t want to give too much information on right now, We’re worried that calling CPS will make the mom not let the kids come down to see us and what’s left of their dads family so we really wanted to have enough evidence before calling in, there’s no really immediate danger, just long term mental and emotional abuse along with medical neglect