I wouldn't go down the route of laxatives just yet tbh, I think it would be best to wait it out and see what happens.
I should warn you though, that this is exactly what my DD did and 6 months on she regularly witholds her poo for days until she absolutely has to go... I think I trained her too early in hindsight and for some reason as soon as she became aware of pooing IYKWIM she started to hold it in.
Are you sure your DD is ready, because to be honest if I could go back, I wouldn't have continued to train her if I'd known this would happen.
This may not be the case with your DD of course, but instead of laxatives you could try ensuring she has lots of fruit, water and fibre to ensure she can poo and it will be soft for her and therefore harder to hold in.
I've been attempting to toilet-train DD1 (2.5) for the past 2.5 days, and she has only wee'd on the toilet once (tiny amount!), and has wee'd several times round the house, on an absorbent mat when sat on the sofa or when I put pull-ups on her. But we've had no poos at all (she's usually pretty regular, one a day, or two the next day to make up if she 'misses' a day)
I'm getting a potty later to try and encourage her to go when watching the tv and/or in the hope that it's less intimidating than the toilet, but should I consider giving her a gentle laxative?
Also, she's in pull-ups when we go out, or she goes to bed - should I have her in knickers?! Help!