I recently sent my Sims on vacation however one of the tourists we met, was a child that back home had not yet been born. After returning home the child remained in the relationship panel but the mother was still pregnant, child did not live with her.

The baby should have been born 2 days ago but mother is now stuck giving birth.
Is there anyway to fix this other than, returning to my pre travel save and shutting of tourists?

This is puzzling. Unborn children aren't sims yet, they don't have names, genders, or anything so it sounds like the game attempted to resolve the foreign (homebound) pregnancy somehow and the mod then made the child into a tourist. From where you are now, can you try using MasterController to Totally Annihilate the child (you will likely have to fiddle with the Homeworld filter to find them) and then use DebugEnabler to force the pregnant mother to give birth?

Its taken most of the day but have managed to get the mother to give birth, a boy again but different name. I returned to an earlier save from when my Sims arrived home. I tried using debug at that time to get her to give birth but although she got the moodlet it was no different than before, so I reloaded again and used mastercontrolled to reset her and then debug and that worked.

The other child still appears in the relationship panel grayed out, like the tourists. It also appears in the inactive family's familytree, I switched over to have a look. I will look in into The Total Annihilation route to prevent further issues.

I have finally located the child, Mastercontoller, Advanced ,Total Annihilate, Alive.

Would adding the child via the add sim route damage my game? As i would prefere that than killing it.

Can't say for sure as I've never experienced this. The child would be a Mini-Sim as they exist now (I think), but with the full collection of mods designed to help protect against the breakage that can happen by adding one to your household, doing that might be okay. Those are ErrorTrap, Overwatch, Traveler, and RelationshipPanel.

But I would be on the lookout for odd things for a while, especially related to the child. It's not clear if they are carrying enough data to become a full-fledged sim if they only ever existed as far as we know as a tourist.

Well I will make a back up save first and then give it a try. Its a child of a inactive family, so will return it to them and see how it goes. (My active Sims do have the option to chat long distance or ask to visit)

Is there a way to stick an age restriction on Register so children can not be tourists?

Not that I'm aware of. A couple of players had problems long ago when occasionally a baby (or toddler) started showing up as tourists because of course they cannot get around town by themselves, at least not very well in the case of toddlers, but that got locked down with Register v80. Children don't usually cause a problem but this one did because they didn't fully exist back in your homeworld, so it's kind of an unusual situation.

Its been a week since the child rejoined the game. I checked in on the family again as I knew the family knew of the child but had never seen child's family tree. It all looked fine all family relationships seem to be flagged correctly. The child attends school does it homework and all the normal things the inactives do.

What sort of issues can adding mini Sims have to the game? I have had a few errors but none relating to the child or there family.

A Mini-Sim is typically a scaled down data representation of a full-fledged sim. All kinds of facets of their personality tend to be "missing" or undefined and, if not enough data is present when they are converted to a resident, they can error out in all kinds of ways because of that. Sounds like this one recovered nicely from what you have described.

The Child has now aged into a Teen without issues, his siblings have all had there birthdays as well. It does appear that the mods managed to save enough, to make him a full sim. Thank you for all your help.