Yes, as long as they're the same gender they can help them. The bathrooms aren't split by age.
CMs however cannot go in and help (they will obviously go in in pairs if a child has an accident etc, just not general toileting), they can assist with buttons and zips and stuff outside but cannot go in.
There is a family bathroom that a parent can go in with their kids too, so if you ever needed to go in and remind/encourage you can use that one together.
Depending on the ship a sibling of the opposite sex may not be able to help (e.g. the family bathroom in the lab on the classics is in the space so the can, but the one in the club is through an extra door so probably not).
Just be careful how you word this in front of certain CMs, 99% (100% of those in yellow) won't care and will encourage the elder kids to help the younger, however, I have seen a manager tell parents that kids shouldn't have to rely on siblings to go to the bathroom and part of being fully potty trained they should be able to fully go by themselves and they were then offered the nursery - very rare but I have seen it happen.